Use shim for legacy client sample_row_keys

The legacy client exposes [sample_row_keys as a method](https://github.com/googleapis/python-bigtable/blob/3648d7000df4a108524e29a9c55639dc92c1c2cc/google/cloud/bigtable/table.py#L741) on the Table class. This should be simple to swap out for the [veneer substitute](https://github.com/googleapis/python-bigtable/blob/3648d7000df4a108524e29a9c55639dc92c1c2cc/google/cloud/bigtable/data/_sync_autogen/client.py#L990)

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](https://github.com/googleapis/python-bigtable/blob/3648d7000df4a108524e29a9c55639dc92c1c2cc/google/cloud/bigtable/data/_sync_autogen/client.py#L1045) (with a [type alias of RowKeySamples](https://github.com/googleapis/python-bigtable/blob/3648d7000df4a108524e29a9c55639dc92c1c2cc/google/cloud/bigtable/data/_helpers.py#L39)). 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