Implement send_custom_request

[silvanshade]

This PR just adds a send_custom_request method similar to send_custom_notification.

[silvanshade]

Any chance this could be merged @ebkalderon? There are a few features in the WebAssembly language server that I will need this for. By the way, if are busy at the moment and could use help with maintenance I'd be happy to help.

[ebkalderon]

Sorry for the delayed review, @silvanshade! I was in the process of searching for a new job and relocating to a different country, so I haven't had much bandwidth to devote to tower-lsp until recently. I just saw a few of your old pull requests, and they look pretty great! I'm not sure if you'd like to resubmit them for review or not?

I'm happy that the lspower fork exists in the meantime to satisfy users. I've recently been pruning my old backlog and am looking to continue maintaining this crate, including finally refactoring the Server and MessageStream architecture to eliminate the risks of stalling caused by src/transport.rs:L98. Honestly, whether you'd like to join forces in one project or let lspower continue in a separate direction is your call and the community's call. I'm down either way!

Finally, I just want to thank you for all the contributions and improvements you've made to tower-lsp thus far. ❤️