Update ECS Credentials Provider to add support for AWS_CONTAINER_AUTHORIZATION_TOKEN_FILE

[waahm7]

Issue #, if available:

Description of changes:
Adds support for AWS_CONTAINER_AUTHORIZATION_TOKEN_FILE with the following priority

1. auth_token parameter
2. AWS_CONTAINER_AUTHORIZATION_TOKEN_FILE (env var which contains absolute path to the token file. The file will be re-read for each call to get credentials.)
3. AWS_CONTAINER_AUTHORIZATION_TOKEN (env var which contains static auth token)

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 78.26087% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 79.54%. Comparing base (0de6b27) to head (ea1dd45).

Files	Patch %	Lines
source/credentials_provider_ecs.c	77.27%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #233      +/-   ##
==========================================
- Coverage   79.55%   79.54%   -0.01%     
==========================================
  Files          33       33              
  Lines        5860     5872      +12     
==========================================
+ Hits         4662     4671       +9     
- Misses       1198     1201       +3     

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