Eglot: Handle LSP progress with Emacs progress reporters (bug#59149)

Eglot: Handle LSP progress with Emacs progress reporters (bug#59149)

Co-authored-by: João Távora <joaotavora@gmail.com>
* lisp/progmodes/eglot.el (eglot-report-progress): New custom variable.
(eglot-lsp-server): New slot for tracking active progress reporters.
(eglot-handle-notification (eql $/progress)): New method.

The LSP spec describes methods for reporting progress on long running
jobs to the client:

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#progress
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#workDoneProgress

This change reports those notifications in the minibuffer as they come
in.  It shows a percent indicator (if the server provides theme), or a
spinner.

This change could open the door for writing a "cancel long running
request" command, which are identified by these progress
notifications.  See
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#window_workDoneProgress_cancel

* doc/misc/eglot.texi (Customizing Eglot): Describe new variable.