[google-auth-library-java] Add DefaultAwsSecurityCredentialsSupplier supporting AWS default credentials chain

**Is your feature request related to a problem? Please describe.**
The current [InternalAwsSecurityCredentialsSupplier](https://github.com/googleapis/google-auth-library-java/blob/v1.23.0/oauth2_http/java/com/google/auth/oauth2/InternalAwsSecurityCredentialsSupplier.java#L52-L55) does not support all AWS authentication methods and depends on an AWSCredentialSource. In many cases, it would be useful to have a built-in implementation that leverages the AWS Default Credentials provider chain.

**Describe the solution you'd like**
A full implementation, similar to the one shown in [this sample](https://github.com/googleapis/google-auth-library-java/blob/73e9e448dfc48de783ee0740c288853de51ac964/samples/snippets/src/main/java/CustomCredentialSupplierAwsWorkload.java#L110), could be added as a separate class (e.g., DefaultAwsSecurityCredentialsSupplier) that uses the default AWS credential resolution logic.

Start with oauth2_http/java/com/google/auth/oauth2/InternalAwsSecurityCredentialsSupplier.java and compare it with samples/snippets/src/main/java/CustomCredentialSupplierAwsWorkload.java. Trace how the sample resolves AWS credentials and define the separate DefaultAwsSecurityCredentialsSupplier around that behavior. Done means the new supplier supports AWS's default credentials provider chain.

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