Investigate how to Expose EndpointContext in Java-Core

[lqiu96]

GAPICs use the EndpointContext to resolve the endpoint and universe domain.

As of now, we are simply duplicating the logic between Gax-Java and Java-Core. We would like to be able to use the EndpointContext inside Java-Core so that we only need to maintain one spot to resolve the endpoint and universe domain.

The issue is that EndpointContext was written with GAPICs in mind and contain some GAPIC specific functionalities.

There are a few open questions that need to be investigated and solved: https://github.com/googleapis/sdk-platform-java/pull/2592#discussion_r1560068306