[BUG] VsCode Credential not reading correct values from the access token cache.

[g2vinay]

Describe the bug
VsCode credential is reading in the token value of 'Azure' account from the VSCode iDE cache. This value is never purged, so cannot be relied upon. This results in following issues:

1. VSCode credential reads token even if no user logged in the IDE.
2. VSCode credential reads in expired tokens, if the user hasn't logged in the VSCode IDE in weeks/months.
3. Azure CLI can never be reached in DAC flow.

To Reproduce

1. Login to VS Code IDE against public cloud.
2. Logout of VSCode IDE.
3. VsCode credential will continue to work.
4. Let the token expire.
5. VsCode credential will read expired token and break the DAC flow.
6. Azure CLI is never reached in DAC credential.

Expected behavior
VSCode credential should throw Credential Unavailable Exception when no user logged into VS code IDE.

How to Fix

1. Update Identity SDK VSCode Cred public cloud env mapping to AzureCloud from Azure.
2. Use AzureCloud as default env instead of Azure in the Identity SDK code for VSCode credential.