This adjusts the ProtocolServer command to default to create a new
connection listening on `localhost:0` and adds a new `ServerMetadata`
details to `~/.lldb/mcp/lldb-<pid>.json` to record information about the
current MCP server.

This can be consumed by the lldb-mcp binary to establish a connection
from an LLM client.

---------

Co-authored-by: Jonas Devlieghere <jonas@devlieghere.com>

When starting an MCP protocol server that uses unix sockets as the
transport, a local `'[0.0.0.0]:0'` file is used instead of the supplied
socket path, e.g:
```
(lldb) protocol-server start MCP accept:///tmp/some/path.sock
MCP server started with connection listeners: unix-connect://[0.0.0.0]:0
(lldb) shell ls '[*'
[0.0.0.0]:0
```

This change makes it so that the URI path is used if the socket protocol
is `ProtocolUnixDomain`:
```
(lldb) protocol-server start MCP accept:///tmp/some/path.sock
MCP server started with connection listeners: unix-connect:///tmp/some/path.sock
```

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.