Bigtable: Admin should have proto utility methods

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.

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