This moves all the generic MCP code into the new ProtocolMCP library so
it can be shared between the MCP implementation in LLDB (built on the
private API) and lldb-mcp (built on the public API).
This PR doesn't include the actual MCP server. The reason is that the
transport mechanism will be different between the LLDB implementation
(using sockets) and the lldb-mcp implementation (using stdio). The goal
s to abstract away that difference by making the server use
JSONTransport (again) but I'm saving that for a separate PR.
