* This adjusts the `Request`/`Response` types to have an `id` that is
either a string or a number.
* Merges 'Error' into 'Response' to have a single response type that
represents both errors and results.
* Adjusts the `Error.data` field to by any JSON value.
* Adds `operator==` support to the base protocol types and simplifies
the tests.
This is a continuation of #152188, which started splitting up the MCP
implementation into a generic implementation in Protocol/MCP that will
be shared between LLDB and lldb-mcp.
For now I kept all the networking code in the MCP server plugin. Once
the changes to JSONTransport land, we might be able to move more of it
into the Protocol library.