Fix STS Credentials Provider to use regional endpoint if possible

[waahm7]

Issue #, if available:
awslabs/mountpoint-s3#861

Description of changes:

• If we can parse region from environment variable or config file, use a regional endpoint instead of global sts endpoint.
• Update STSWebIdentity to look for the AWS_REGION variable first as well.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 82.47423% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 80.22%. Comparing base (140652a) to head (c29497d).

Files	Patch %	Lines
source/credentials_provider_sts.c	81.48%	10 Missing ⚠️
source/credentials_utils.c	78.57%	6 Missing ⚠️
source/credentials_provider_sts_web_identity.c	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   79.91%   80.22%   +0.30%     
==========================================
  Files          33       33              
  Lines        5921     5958      +37     
==========================================
+ Hits         4732     4780      +48     
+ Misses       1189     1178      -11     

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[DmitriyMusatkin]

debatable: is it better to hide the buf as impl detail and return string instead? dont have to deal with cleaning buf, worrying if they pass an existing buf, etc...

[waahm7]

Thanks, updated. That's much better.