CRT-Java `CredentialsProvider` should be an interface

[kggilmer]

tl;dr is that crt-java credentials provider is an abstract class that is tightly coupled to the JNI native bindings and prevents users from supplying a pure JVM implementation of a credentials provider.

This isn't a huge deal yet but it would prevent e.g. customers or the Kotlin SDK team from writing a custom provider implementation.

See:
awslabs/aws-crt-java#252

internal id: 175851816

[aajtodd]

NOTE: that this may not be that big of a deal in practice because the SDK doesn't pass the credential provider to through the signing config. Instead it is called directly and then the credentials returned are passed to the signing config.