Always default default the JWT audience to the primary bigtable.googleapis.com authority

[igorbernstein2]

Currently the JWT audience is derived from the endpoint. This works well when the endpoint is bigtable.googleapis.com. However when a different endpoint is used to reach cloud bigtable, this doesnt work as well. The EnhancedStubSettings has a way to new mappings between endpoints and authorities via JwtAudienceMapping, by default this has a mapping for batch-bigtable.googleapis.com.

However with psc, having multiple endpoints targeting the same authority is the norm. So the mapping should ignore the endpoint and simply default to bigtable.googleapis.com unless there is an override