Future Improvements for "X.509 WIF mTLS-transport support in Java Gax lib"

[andyrzhao]

This is a follow-up for #3851 to address some secondary code organization concerns discovered during PR review. Details are in the linked PR comments, but to summarize a few key points:

1. EndpointContext, GrpcChannelProvider, and HttpJsonChannelProvider each instantiate its own mTLS provider today using a similar but duplicated flow - this could potentially be refactored so that a single mTLS provider can be shared between EndpointContext and the Channel providers.

2. A related concern is to address is the nullability of "CertificateBasedAccess" helper in EndpointContext to make it non-Nullable without breaking existing tests.