Helpers for server-specific functionality

[ArsenArsen]

Hi!

Certain servers (most notably for me JDTLS) have server-specific commands that are very useful, for instance, JDTLS specifically has a java/projectConfigurationUpdate notification that should be sent to it after editing build system files.

I am content with invoking this command manually, but currently, Eglot has no way of doing so out of the box.

I've added the following to my init.el:

(defun jdtls-reload-project-config (&optional server)
  "Tell jdtls to reload the project configuration.  Useful after build system
changes."
  (interactive)
  (let ((truename (file-truename (or buffer-file-name
                                     (ignore-errors
                                       (buffer-file-name
                                        (buffer-base-buffer))))))
        (srv (or server (eglot-current-server))))
    (jsonrpc-notify srv
                    :java/projectConfigurationUpdate
                    `(:uri ,(eglot-path-to-uri truename :truenamep t)))))

I'm not sure whether it is correct, and I copied the :uri logic from eglot.el.

I see that other LSP implementations usually have a language-specific package which contains helpers like these as well as server configuration. While I don't think that eglot should necessarily take this specific approach, it seems to me that server-specific work is sometimes necessary. Namely, server-specific commands like the above seem relatively low-cost but helpful.

Could eglot provide helpers akin to the above? As well as a method to distinguish whether they are applicable (say, a server field called serverHelpers or such that these helpers would check against).

What I imagined for the above is something like an interactive command which either takes the current server or, if none is present or prefix is passed, asks for which JDTLS server to send the notification to. It'd also be good if the user could pass a server when calling the function noninteractively, so that a specific argument can be passed.