Renames the tailnet.Coordinator to represent both v1 and v2 APIs, so that we can use this interface for the main atomic pointer.

Part of #10532

Part of #10532

Adds a method to validate a requested version of the tailnet API

Part of #10532

DRPC transport over yamux and in-mem pipes was previously only used on the provisioner APIs, but now will also be used in tailnet.  Moved to subpackage of codersdk to avoid import loops.

Part of #10532

Adds a tailnet ClientService that accepts a net.Conn and serves v1 or v2 of the tailnet API.

Also adds a DRPCService that implements the DRPC interface for the v2 API.  This component is within the ClientService, but needs to be reusable and exported so that we can also embed it in the Agent API.

Finally, includes a NewDRPCClient function that takes a net.Conn and runs dRPC in yamux over it on the client side.

closes #10532

Adds v2 support to the /coordinate endpoint via a query parameter.

v1 already has test cases, and we haven't implemented v2 at the client yet, so the only new test case is an unsupported version.