Use shim for legacy client sample_row_keys

[daniel-sanche]

The legacy client exposes sample_row_keys as a method on the Table class. This should be simple to swap out for the veneer substitute

The one complication should be the return types; the legacy client seems to return the gapic response directly, while the veneer client unpacks it into a tuple list (with a type alias of RowKeySamples). We could re-build the proto object to keep things consistent, or possibly change the return type as a breaking change. Depends on what we see about typical usage