[Internal] Decouple serving and oauth2 package (#1110)

## What changes are proposed in this pull request?

Package `oauth2` contains a `DataPlaneService` that is not intended for
users to use directly. This service is not auto-generated and is only
used in the `serving` package. To remove the dependency between the
`serving` package and the `oauth2` package, this PR moves the
`DataPlaneService` from the `oauth2` to the `serving` package.

## How is this tested?
Existing unit tests

.codegen/_openapi_sha

@@ -1 +1 @@
-a6a317df8327c9b1e5cb59a03a42ffa2aabeef6d
+1668b0db17e23605f8c9d29fb3b674c01590732d

.gitattributes

@@ -77,9 +77,11 @@ experimental/mocks/service/marketplace/mock_provider_provider_analytics_dashboar
 experimental/mocks/service/marketplace/mock_provider_providers_interface.go linguist-generated=true
 experimental/mocks/service/ml/mock_experiments_interface.go linguist-generated=true
 experimental/mocks/service/ml/mock_model_registry_interface.go linguist-generated=true
+experimental/mocks/service/oauth2/mock_account_federation_policy_interface.go linguist-generated=true
 experimental/mocks/service/oauth2/mock_custom_app_integration_interface.go linguist-generated=true
 experimental/mocks/service/oauth2/mock_o_auth_published_apps_interface.go linguist-generated=true
 experimental/mocks/service/oauth2/mock_published_app_integration_interface.go linguist-generated=true
+experimental/mocks/service/oauth2/mock_service_principal_federation_policy_interface.go linguist-generated=true
 experimental/mocks/service/oauth2/mock_service_principal_secrets_interface.go linguist-generated=true
 experimental/mocks/service/pipelines/mock_pipelines_interface.go linguist-generated=true
 experimental/mocks/service/provisioning/mock_credentials_interface.go linguist-generated=true