feat: Add support for custom language servers

[LeeroyDing]

Implementation of #285

[LeeroyDing]

NOT manually tested yet!

[MischaPanch]

It won't exactly work like that since initialization_options are not static - they need the process_id and the repo_uri. Also the checking for custom_ls.yml is not appropriate (there may be several custom LS)

If you don't need it for now, we can leave the PR open and later build on it. I think a better way to allow users to start custom LS is by allowing them to make an actual python implementation of an LS using whatever startup command they want (i.e. a python module, not a json), and let serena pick it up automatically. So something like a command register_ls which modifies the list of supported languages and the LS creation, and the user can provide a path to that command which is executed when the server is spun up. Something like that - mainly that users write python and not json for getting custom ls support.

[LeeroyDing]

I did find a use for it, will take your suggestion and try it at some point in the next couple of weeks