Allow custom s3 endpoint_url for use with boto3[crt]

[masudio]

Description

The tl;dr is that boto3 has an option to use a faster underlying package called CRT (Common RunTime) for far greater S3 throughput. However, the integration with the underlying library is not complete for custom S3 endpoints (e.g. a company hosts an internal S3 API).

In addition, there is a bug this diff fixes where 'compare_identity' function doesn't treat "" and None as equal - this also prevents users from using CRT with boto3 with a custom endpoint_url.

More concretely, this particular diff does these pieces:

• change to let crt request serializer factory method use a custom endpoint_url
• there’s a bug in an equality comparison where “” == None needs to evaluate to true in context of credential providers

Testing

Added unit tests to cover the changes.