Bigtable: Admin should have proto utility methods

[sduskis]

There are some complex protobuf objects. Here are some possibilities where utilities might come in handy:

• any message with "oneof" gets confusjng, since users set multiple values (AppProfile, DropRowRangeRequest, ModifyColumnFamiliesRequest)
• Instance messages are complex
• PartialUpdateInstanceRequest use google.protobuf.FieldMask which is quite confusing
• CheckConsistency is used for for polling, so it's ideal having a utility for this kind of functionality
• "TableExists" type functionality should use ListTables instead of GetTable, since it's less expensive. Users should have a clear API that produces the most efficient calls.

[yihanzhen]

I'm not sure about priority. Feel free to bump if necessary.

[igorbernstein2]

This is currently being worked on: https://github.com/GoogleCloudPlatform/google-cloud-java/pull/3395

[igorbernstein2]

I'm still in the process of cleaning things up, but the wrappers have landed.