Do rust's rls hack properly with new class-based api

Do rust's rls hack properly with new class-based api

* eglot.el (eglot-server-ready-p): New API method.
(eglot-handle-request, eglot-handle-notification): New
defgeneric's.
(eglot--ready-predicates, eglot--server-ready-p): Remove.
(eglot--async-request): Call eglot-server-ready-p.
(eglot--request): Tweak comment.
(eglot--rls-probably-ready-for-p): Remove.
(eglot-server-ready-p eglot-rls): Adapts earlier
eglot--rls-probably-ready-for-p.
(eglot-handle-notification eglot-rls): Specialize to eglot-rls.
(eglot-rls): New eglot-lsp-server subclass.

* eglot-tests.el (auto-detect-running-server)
(auto-reconnect, basic-completions)
(hover-after-completions): Use eglot--interactive