fixes sts creds provider env var resolution

[sbiscigl]

Issue #, if available:

#3600

Description of changes:

Fixes two problems:

• The STSWebIdentityCredentialsProvider used a typo-ed version of two env vars, AWS_IAM_ROLE_ARN and AWS_IAM_ROLE_SESSION_NAME. this changes the provider to use the correct AWS_ROLE_ARN and AWS_ROLE_SESSION_NAME . In addition it provides backwards compatibility to the old typo-ed names.
• The function LoadConfigFromEnvOrProfile will call ToLower on any environment variable loaded, which is incorrect for the env var AWS_IAM_ROLE_ARN as arns can contain case sensitive values. This is likely a greater issue across other configurations, but fixing for the STS web identity provider for now.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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