Eglot: use Eldoc in eglot-hierarchy-mode

Eglot: use Eldoc in eglot-hierarchy-mode

Use it to show details of the thing at point, like the full
signature and the locus of the definition.

* lisp/progmodes/eglot.el: Add optional arg MODE.
(eglot--hierarchy-source-major-mode): New defvar.
(eglot--hierarchy-1): Adjust.
(eglot-hierarchy-mode): Set eldoc stuff.
(eglot-hierarchy-center-on-node): Cosmetic fix.
(eglot-hierarchy-detail-eldoc-function)
(eglot-hierarchy-locus-eldoc-function): New eldoc functions.

(cherry picked from commit 3fd0b802de20dc83b5d5236b6d458df73c9d4e77)