Expand Server interface

Currently there's no way to make a cross network Server library, as each network lib returns it's own specific type for `SomethingClient SomethingServer::available()`.

We need to introduce another API library developers can use to make a non-board specific networking Server lib.

Review the existing network libraries and their SomethingServer::available() return types, then determine the common API needed for a cross-network Server library. The work is done when developers can use the new API without relying on a board-specific server type.

Written by the indexing model from the issue text.