CredentialsProvider should be an interface

[aajtodd]

Currently CredentialsProvider is an abstract base class that expects sub-classes to be native implementations.

This prevents a library from providing a pure Java/Kotlin implementation of a credentials provider.

This should be refactored to an interface and the abstract base class can be made internal. The implementation will need to provide a bridge that wraps externally provided providers that proxies to C and back.

[bretambrose]

On the surface, this seems very reasonable. We may not get to it for some time though.

[jmklix]

Closing. Use delegate credentials provider for similar functionality