Investigate the impact of changing ClientConext's getEndpoint() on downstream Handwritten Libraries

[lqiu96]

Modifying this value:

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/rpc/ClientContext.java

Line 259 in 2cb0f9e

.setEndpoint(settings.getEndpoint())

to be the resolved endpoint from the EndpointContext may break downstream handwritten libraries.

If there is no impact, try to change the value.
If there is impact and ClientContext needs to return the resolvedEndpoint, look into exposing a few field to return

[lqiu96]

ClientContext's getEndpoint() returns the StubSettings' getEndpoint() value which is the resolved endpoint. When data is passed between ClientContext and StubSettings, all the the endpoint information is passed between the two. StubSettings will recreated the EndpointContext with the original values so any user configurations will not be lost.

Going to close this issue as this was done in #2715