feat: added configuration variables to handle EC2 metadata service

[mightyshazam]

Description

Rusoto and the AWS SDK have different behavior when interacting with the ec2 metadata service. Despite the lower default timeout in the AWS SDK, it still takes longer to abandon the IMDS credential provider. This PR add a setting AWS_EC2_METADATA_DISABLED to disable the provider. It can be provided as an environment variable or isolated to the s3 storage settings. Additionally, I added AWS_EC2_METADATA_TIMEOUT to represent the timeout for the imds credential provider in milliseconds with a default value of 100.
This does not completely eliminate the regression, but it provides opt-in/opt-out options to mitigate the issues in exceptional cases.

Related Issue(s)

• closes Very slow s3 connection after 0.16.1 #2377

Documentation

Rust SDK Environment Variables

[echai58]

Hi, bump on reviewing this 🙂 , this is blocking my ability to upgrade past 0.16.1 which has some important bug fixes/features.

[echai58]

@mightyshazam Hi, I tried running against a local build with this commit, and it seems like passing "AWS_EC2_METADATA_DISABLED": "true" via the storage_options still raises the failed to load region from IMDS err warning and slow reads, whereas setting the environment variable prevents this behavior.