NewInstanceCredentialsCache returns *aws.CredentialsCache

[tinnywang]

Summary

Fix NewInstanceCredentialsCache so that it actually wraps the credentials provider in a credentials cache. This is a followup to https://github.com/aws/amazon-ecs-agent/pull/4424#discussion_r1844604109:

https://github.com/aws/amazon-ecs-agent/commit/a9b6e6de25541353ec5c7c6d640ed21374a08ca7 updates the constructor to wrap the provider in a cache and renames InstanceCredentialsProvider to InstanceCredentialsCache.

As you can see in the code snippets below, the returned provider is not wrapped in a cache. https://github.com/aws/amazon-ecs-agent/blob/c24cdaea3e30649fdfbde4906ca964bad49f3126/ecs-agent/credentials/providers/instance_credentials_provider_linux.go#L41-L46

https://github.com/aws/amazon-ecs-agent/blob/c24cdaea3e30649fdfbde4906ca964bad49f3126/ecs-agent/credentials/providers/instance_credentials_provider_windows.go#L76-L78

Implementation details

• Rename InstanceCredentialsCache back to InstanceCredentialsProvider (but leave the constructor NewInstanceCredentialsCache because it returns a credentials cache).
• Update NewInstanceCredentialsCache to wrap InstanceCredentialsProvider in a cache and return *aws.CredentialsCache.

  return aws.NewCredentialsCache(&InstanceCredentialsProvider{...})

• Change the type of the ecsAgent.credentialsCache field from aws.CredentialsProvider to *aws.CredentialsCache. Foresight has shown me that other parts of our codebase will use ecsAgent.credentialsCache and forcefully expire cached credentials, for which they'll need CredentialsCache.Invalidate. The aws.CredentialsProvider interface does not have an Invalidate method.

Testing

Github checks pass.

New tests cover the changes: no

Licensing

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