feat: Remove setting the default endpoint in StubSettings

[lqiu96]

What does this do

1. This change removes the concept of a having a default endpoint. For GAPICs, the EndpointContext will resolve the endpoint. If there are no user configurations, the endpoint will resolve to {serviceName}.googleapis.com.
2. StubSettings' getEndpoint() will return the user set endpoint. If the user does not set an endpoint, the method will return the endpoint set by the service.

Showcase

Showcase Clients do not have a valid serviceName and cannot construct an endpoint via the EndpointContext without an explicit endpoint input. To run these locally, we must configure the endpoint explicitly to point to localhost:7469 for gRPC and http://localhost:7469 for httpjson.

Google Cloud Libraries

For Google Cloud client libraries, the service name is stored in the EndpointContext and parsed from the proto or service yaml file.

BEGIN_COMMIT_OVERRIDE
feat: Do not set the default endpoint in StubSettings
feat: StubSettings' getEndpoint() will return the service's pre-configured endpoint if there are no user configurations
END_COMMIT_OVERRIDE

[blakeli0]

Removing setting the default endpoint maybe breaking for customers, especially for some handwritten libraries like Bigtable, can you please verify? If yes, I don't think we want to change it until getResolvedEndpoint() is introduced publicly. Then we can probably move EndpointContext to StubSettings and do something like setEndpoint(endpointContext.getResolvedEndpoint()).

[sonarqubecloud]

Quality Gate failed for 'gapic-generator-java-root'

Failed conditions

54.8% Coverage on New Code (required ≥ 80%)
44.8% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud

[sonarqubecloud]

Quality Gate failed for 'java_showcase_integration_tests'

Failed conditions

30.0% Coverage on New Code (required ≥ 80%)
54.8% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud

[blakeli0]

LGTM. Before merging, please update the PR title and description to reflect the new changes, and change the type of the PR from chore to feat.