Universe Domain Support

Tracking Bug for all Universe Domain Service Library tasks

### Phase 1 Unblocking Service/Generator Work

- [x] #13116
- [x] #13117
- [x] #13118
- [x] #13119

### Phase 2 Service/Generator Work

- [x] #13170
- [x] #13303
- [x] #13300
- [x] #13272
- [x] #13293
- [x] #13295
- [x] #13294
- [x] #13296
- [x] #13301
- [x] #13302
- [x] #13343
- [x] #13344
- [x] #13345
- [x] #13474
- [x] Add support for GOOGLE_CLOUD_UNIVERSE_DOMAIN env var in `DetermineServiceEndpoint`
- [x] #13472
- [x] #13473
- [x] #13122
- [x] #13233
- [x] Update MakeConnection and stub creation functions to use Options modified by stub creation functions instead of just a copy of the Options provided (REST).

### Phase 3 Integration Tests/Documentation

- [ ] Update `PopulateCommonOptions` to no longer set `EndpointOption`.
- [ ] Set up pr/ci builds that execute against testing environment
- [ ] Add integration tests for Bigtable
- [ ] Add integration tests for PubSub
- [ ] Add integration tests for PubSubLite
- [ ] Add integration tests for Spanner
- [ ] Add integration tests for Storage (gRPC)
- [ ] Add integration tests for Storage (REST)
- [ ] Add integration tests for Compute (to test generated REST service)
- [ ] Add integration tests for KMS (to test generated gRPC service)
- [ ] Add in-depth guide to using `UniverseDomain` in docs/ directory which refers to all the relevant code samples
- [ ] Add sample code for retrieving the `UniverseDomainOption` from an instance of a Connection
- [ ] Add sample code for creating a Client using `AddUniverseDomainOption` and checking the `UniverseDomainOption` value
- [ ] Add sample code for creating a Client using `AddUniverseDomainOption` without checking the `UniverseDomainOption` value
- [ ] Add sample code for creating a Client using `GetUniverseDomainOption`
- [ ] Add sample code for setting an explicit `UniverseDomainOption` used in Connection creation
- [ ] Add sample code for creating a custom `UniverseDomainRetryPolicy` and setting it the Options, along with using it with either `AddUniverseDomainOption` or `GetUniverseDomainOption`
- [ ] #13482

### Phase 4 Make Universe Domain support available

- [ ] #13121
- [ ] Remove `ExperimentalTag` from `AddUniverseDomainOption` and `GetUniverseDomain` functions.