Add BigtableDataClient to legacy client init #15298

To enable the shim, we need to let the legacy client hold an instance of the hand-written BigtableDataClient.

The data client uses background threads to manage automatic channel refresh, which are cleaned up when close() is called. Code written for the legacy client likely doesn't call close the same way. We should disable background refresh for the wrapped client (and maybe expose it to end-users as well?)

Note: Consider whether the wrapped data client should use a different version string for analytics

From go/bigtable_legacy_deprecation:

Old API

• Users first create a Client, associated with a GCP project
• Then, they call client.instance(instance_id) to create an Instance associated with a a bigtable instance_id
• Then, they call instance.table(table_id, app_profile_id) to create a new Table associated with a bigtable table_id
• They can then call methods on the table instance to execute bigtable rpcs

Replacement

• We can create a BigtableDataClient when creating a legacy client, and hold it in a new internal client._data_client variable
• We can create a data Table when creating a legacy table, and hold it in a new internal table._data_table variable