Deprecate DataClient public member function

[dbolduc]

What?:
We want to deprecate and schedule for deletion these member functions:

google-cloud-cpp/google/cloud/bigtable/data_client.h

Lines 63 to 84 in 8e9b821

	/**
	* Return a new channel to handle admin operations.
	*
	* Intended to access rarely used services in the same endpoints as the
	* Bigtable admin interfaces, for example, the google.longrunning.Operations.
	*/
	virtual std::shared_ptr<grpc::Channel> Channel() = 0;
	/**
	* Reset and create new Channels.
	*
	* Currently this is only used in testing. In the future, we expect this,
	* or a similar member function, will be needed to handle errors that require
	* a new connection, or an explicit refresh of the credentials.
	*/
	virtual void reset() = 0;
	/**
	* The thread factory this client was created with.
	*/
	virtual google::cloud::BackgroundThreadsFactory
	BackgroundThreadsFactory() = 0;

Why?:
We are removing these because:

• Only the client library should need this level of control over the gRPC channels.
• We want to perform work using the Connection layer (which offers customers better mocks, unified authentication credentials support, etc.). We do not want to maintain the legacy code path (these functions), when there is a preferred alternative. See Proposal: modernize the Bigtable internals #5934.

How?:
We still call some of these. We can create private implementations where necessary. Our code can call the private implementations directly. The public member functions would also call the private implementations.

[dbolduc]

The functions are deprecated. They can be deleted (and the class can be simplified) on 2023-05-01.

[coryan]

These are deprecated, scheduled for removal around 2023-07.