Reduce redundant logging during ChainedTokenCredential.get_token()

[chlowell]

This PR closes #18972 by reducing redundant logging during ChainedTokenCredential.get_token() and, by inheritance, DefaultAzureCredential.get_token(). The rationale is that when get_token() fails for a credential chain, it logs an aggregate of the inner credentials' error messages, making those credentials' logging of same redundant. Similarly, when get_token() succeeds, users will seldom find error messages from unavailable inner credentials interesting. On the contrary, those messages can be confusing: "Why do I see half a dozen error messages when authentication succeeds?"

Here's the diff for DefaultAzureCredential failure given typical logging configuration (filtering DEBUG messages):

[INFO] No environment configuration found.
[INFO] ManagedIdentityCredential will use IMDS
-[WARNING] EnvironmentCredential.get_token failed: EnvironmentCredential authentication unavailable. Environment variables are not fully configured.
-[INFO] DefaultAzureCredential - EnvironmentCredential is unavailable
[INFO] No response from the IMDS endpoint.
-[WARNING] ImdsCredential.get_token failed: ManagedIdentityCredential authentication unavailable, no managed identity endpoint found.
-[WARNING] ManagedIdentityCredential.get_token failed: ManagedIdentityCredential authentication unavailable, no managed identity endpoint found.
-[INFO] DefaultAzureCredential - ManagedIdentityCredential is unavailable
-[WARNING] SharedTokenCacheCredential.get_token failed: SharedTokenCacheCredential authentication unavailable. No accounts were found in the cache.
-[INFO] DefaultAzureCredential - SharedTokenCacheCredential is unavailable
-[WARNING] VisualStudioCodeCredential.get_token failed: Failed to get Azure user details from Visual Studio Code.
-[INFO] DefaultAzureCredential - VisualStudioCodeCredential is unavailable
-[WARNING] AzureCliCredential.get_token failed: Please run 'az login' to set up an account
-[INFO] DefaultAzureCredential - AzureCliCredential is unavailable
-[WARNING] AzurePowerShellCredential.get_token failed: Please run "Connect-AzAccount" to set up account
-[INFO] DefaultAzureCredential - AzurePowerShellCredential is unavailable
[WARNING] DefaultAzureCredential failed to retrieve a token from the included credentials.
Attempted credentials:
        EnvironmentCredential: EnvironmentCredential authentication unavailable. Environment variables are not fully configured.
        ManagedIdentityCredential: ManagedIdentityCredential authentication unavailable, no managed identity endpoint found.
        SharedTokenCacheCredential: SharedTokenCacheCredential authentication unavailable. No accounts were found in the cache.
        VisualStudioCodeCredential: Failed to get Azure user details from Visual Studio Code.
        AzureCliCredential: Please run 'az login' to set up an account
        AzurePowerShellCredential: Please run "Connect-AzAccount" to set up account

Similarly, DefaultAzureCredential success:

[INFO] No environment configuration found.
[INFO] ManagedIdentityCredential will use IMDS
-[WARNING] EnvironmentCredential.get_token failed: EnvironmentCredential authentication unavailable. Environment variables are not fully configured.
-[INFO] DefaultAzureCredential - EnvironmentCredential is unavailable
[INFO] No response from the IMDS endpoint.
-[WARNING] ImdsCredential.get_token failed: ManagedIdentityCredential authentication unavailable, no managed identity endpoint found.
-[WARNING] ManagedIdentityCredential.get_token failed: ManagedIdentityCredential authentication unavailable, no managed identity endpoint found.
-[INFO] DefaultAzureCredential - ManagedIdentityCredential is unavailable
-[WARNING] SharedTokenCacheCredential.get_token failed: SharedTokenCacheCredential authentication unavailable. No accounts were found in the cache.
-[INFO] DefaultAzureCredential - SharedTokenCacheCredential is unavailable
-[INFO] DefaultAzureCredential - VisualStudioCodeCredential is unavailable
-[WARNING] AzureCliCredential.get_token failed: Please run 'az login' to set up an account
-[INFO] DefaultAzureCredential - AzureCliCredential is unavailable
-[INFO] AzurePowerShellCredential.get_token succeeded
[INFO] DefaultAzureCredential acquired a token from AzurePowerShellCredential

Inner credentials still log these messages, however they do so at DEBUG level when they're invoked by ChainedTokenCredential, using a ContextVar to determine whether that's the case. Unfortunately, ContextVar was added in Python 3.7. For Python 3.6, we have a backport. The nearest equivalent available to 2.7 is threading.local, which isn't concurrency-safe. Another alternative that works everywhere is to pass around an internal keyword argument, but 🤮

In the end, because this isn't a functional or API change, I don't mind excluding 2.7. Similarly, if the backport is unacceptable, I observe that 3.6 will be EOL in 6 months. But that's just my opinion, please feel free to change my mind or propose alternative solutions 😄

[mccoyp]

I think this is a reasonable solution -- at least, I can't think of a better one. I feel bad excluding 2.7, but I don't think the necessary work to support it would be justifiable when this is a cosmetic change, albeit a really nice one.

I do wonder if it would be good to include a note in the changelog that these changes are available on 3.6 if you pip install the backport

[chlowell]

It turns out the backport doesn't work correctly in all cases and likely never will given the impending 3.6 EOL (see MagicStack/contextvars/issues/2). I'll stick with 3.7+ here and recommend upgrading Python to anyone running 3.6 who really wants this change.