Disable workDoneProgress if defcustom is nil

Disable workDoneProgress if defcustom is nil

There is no need to receive the $/progress notifications from the server
if we don't want to render them. Because they are effectively ignored
when eglot-report-progress is nil we'd rather not waste cycles on serde
of the messages.

* lisp/progmodes/eglot.el (eglot-client-capabilities): use value from
defcustom to decide whether or not to advertise to server.

(cherry picked from commit a33e7c0286c2a9c8af1c96db42f5c0c37611c9db)