Currently, the server keeps running until we call Stop from its dtor in
the static destruction chain. This is too late: the server should stop
when the plugin gets terminated.

Rather than having one MCP server per debugger, make the MCP server
global and pass a debugger id along with tool invocations that require
one. This PR also adds a second tool to list the available debuggers
with their targets so the model can decide which debugger instance to
use.

This PR adds an MCP (Model Context Protocol ) server to LLDB. For
motivation and background, please refer to the corresponding RFC:
https://discourse.llvm.org/t/rfc-adding-mcp-support-to-lldb/86798

I implemented this as a new kind of plugin. The idea is that we could
support multiple protocol servers (e.g. if we want to support DAP from
within LLDB). This also introduces a corresponding top-level command
(`protocol-server`) with two subcommands to `start` and `stop` the
server.

```
(lldb) protocol-server start MCP tcp://localhost:1234
MCP server started with connection listeners: connection://[::1]:1234, connection://[127.0.0.1]:1234
```

The MCP sever supports one tool (`lldb_command`) which executes a
command, but can easily be extended with more commands.