Move google-api-services-storagetransfer to the new com.google.cloud library

[danagu]

Current api jar is:

com.google.apis:google-api-services-storagetransfer

In order to manage GCP "JOBS" in a better way, moving to some mutual interface of jobs (such as in BigQuery and com.google.cloud) can be very helpful.

[vam-google]

Cloud Storage Transfer Service is essentially a separate service and is not part of our current Storage Client.

I agree that the storage and storage transfer clients are tightly related and it would be nice to have them in same place, but as of now (mostly for historical reasons) they are two very different things and merging them in a proper manner would require significant amount of work (just putting jars together or porting code of google-api-services-storagetransfer to google-cloud-java/google-cloud-storage is not an option here).

[garrettjonesgoogle]

We are handling requests for supporting additional APIs with an internal mechanism. Closing out this issue.