transcoder: Add Long-Running Operation (LRO) Support to the Transcoder API

## Is your feature request related to a problem? Please describe.
We would appreciate it if the Transcoder API supported the long-running operations pattern. It seems like the GetJob method could be adapted for this purpose.

## Describe the solution you'd like
The CreateJob method should return a standard google.longrunning.Operation object instead of a Job object.
## Describe alternatives you've considered
The only alternative is the current method: manually implementing a polling loop that calls GetJob periodically until the job's state becomes Job_SUCCEEDED or Job_FAILED. This works, but it adds boilerplate code to every implementation and shifts the burden of managing polling onto the developer.

## Additional context
https://google.aip.dev/151