Fix: Respect aws.endpointUrlSTS system property in StsCredentialsProvider

[akitnk]

This is a fix for #6252

Motivation and Context

StsWebIdentityCredentialsProvider does not support properties to configure custom STS endpoint (like aws.endpointUrlSTS and AWS_ENDPOINT_URL_STS). Due to this problem, StsWebIdentityCredentialsProvider cannot access custom STS endpoint even if it is configured using those properties.

Modifications

Respect aws.endpointUrlSTS system property

Testing

Tested locally and confirmed it works as expected

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.
• My change is to implement 1.11 parity feature and I have updated LaunchChangelog ... No. Because 1.x does not support the service specific environment variable/property.

License

• I confirm that this pull request can be released under the Apache 2 license

[akitnk]

Could someone give a comment on this? This issue needs to be fixed when user wants to use IRSA feature on EKS or other K8s platform in the environment which does not support public STS endpoint.
https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html