Introduce eglot-handle-request and eglot-handle-notification as api

Introduce eglot-handle-request and eglot-handle-notification as api

* eglot.el (eglot--process-receive): Call eglot-handle-request and
eglot-handle-notification.
(eglot-handle-notification, eglot-handle-request): New generic
functions.
(eglot--server-window/showMessage)
(eglot--server-window/progress)
(eglot--server-telemetry/event, eglot--server-window/logMessage):
Convert to eglot-handle-notification.
(eglot-handle-request, eglot--server-client/registerCapability)
(eglot--server-client/unregisterCapability)
(eglot-handle-request): Convert to eglot-handle-request.