handwritten layer didn't handle mtls correctly

[arithmetic1728]

The auto-generated layer (google/pubsub_v1) has mtls logic. This client creates a mtls channel based on environment variable settings and existence of device certificate.

However, the handwritten layer (google/cloud/pubsub_v1) creates a channel itself with mtls logic, so the mtls feature is completely missing.

[arithmetic1728]

It seems the easiest solution is (using publisher client as an example):

Create an PublisherClient instance in the handwritten publisher client constructor. PublisherClient is from the auto-generated layer, and it uses the mtls logic to determine the right api_endpoint and ssl_credentials. We then extract the api_endpoint and ssl_credentials from its transport. The handwritten publisher client can then use the ssl_credentials to create the channel, and use the api_endpoint as the target.

This needs a bit of work from python microgenerator to expose api_endpoint and ssl_credentials from the transport class.