Support debugging via SSH

This is different from remote-debugging, which needs a gdbserver on the other side.
It effectively means to ssh into a machine (possibly with client hoping), start GDB there and communicate with this GDB via SSH.

Everything that GDB sees is the ssh machine (using all native paths, having access to all local binaries and debug symbols); to see everything on the "client" side the sources (or a copy of those) need to be accessible to the client. The GDB side _can_ have those, but would only _need_ it to run `list` and `search` commands (because it operates on debug symbols in all other places).
Especially with a copy of the code the paths between the ssh machine and the local machine may be different, so the extension may need to do "translation" using a `SourceFileMap` array (when there's a UI-set breakpoint on the client (which always happens with the "local" paths), the path has to be translated to the "client view"). Similar when GDB tells "I've stopped on this line in that source" the path has to be translated in the other direction to be able to show it in the UI.