Update interface for providing credentials to CRT

[kyleknap]

This is a port of this PR: https://github.com/aws/aws-cli/pull/8334

The two main changes are:

• Update the CRT S3 client factory to accept a CRT credential provider instead of a botocore credential provider. Under the hood, the S3 client only accepts a CRT credential provider. So, this provides more flexibility in being able to provide other CRT credenital providers directly instead of being forced to the botocore credential provider interface
• Update the botocore to CRT credentials adapter interface to only accept botocore credential objects instead of credential providers. In general, the credentials object is more accessible than the provider; it can be retrieved at the session level and is what is passed into clients. Also, this change avoids a limitation where the load_credentials() method on the credential provider cannot be called more than twice for some configurations (e.g. assume role from profile), which can be an issue if you create both a botocore client and CRT S3 client.

[codecov-commenter]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (01aaaf3) 83.85% compared to head (a86f7cf) 84.23%.

Files	Patch %	Lines
s3transfer/crt.py	0.00%	12 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## crt-enhancements #283 +/- ## ==================================================== + Coverage 83.85% 84.23% +0.38% ==================================================== Files 16 16 Lines 2768 2760 -8 ==================================================== + Hits 2321 2325 +4 + Misses 447 435 -12 ```

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