Non-Blocking Credentials Providers

For cross-cloud workloads, generating AWS credentials can entail [a few different network calls](https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_providers_oidc.html).

The interface `AwsCredentialsProvider` provides a single method (`resolveCredentials`) that returns an `AwsCredentials` synchronously. Although implementations will likely cache credentials, the initial lookup will still block here.

Would it be possible to provide an async AwsCredentialsProvider?