chore: factor out per operation settings into a separate class (#2796)

Currently Stubs get access to both BigtableClientContext and EnhancedStubSettings. And since BigtableClientContext is derived from EnhancedStubSettings there is quite a bit of overlap settings which makes it hard to keep them consistent. This PR builds towards fixing this by removing access to EnhancedStubSettings and replacing it with disjoint derivative classes. 

This PR moves all of the per-op settings into a separate class. The next step will be to copy the remaining bits to BigtableClientContext and have EnhancedBigtableStub depend solely on BigtableClientContext and ClientOperationSettings.

Author: igorbernstein2