Allow the client to specify voicemail transcript id's

This is needed for rating transcription quality since we don't
persist the transcript id.  The id we generate is just the md5
hash of the voicemail audio data.

Bug: 68712148
Test: manual and updated unit tests
PiperOrigin-RevId: 174527907
Change-Id: I7a29bf5a96447129fc1437074f20ecebcdb1f43b