Allow eglot-workspace-configuration to be a plist

Allow eglot-workspace-configuration to be a plist

Suggested-by: Augusto Stoffel <arstoffel@gmail.com>
* NEWS.md: Mention change.

* README.md (eglot-workspace-configuration): Update yet again.  Update
examples to use pylsp.

* eglot.el (eglot--workspace-configuration-plist): Noop if already
a plist.
(eglot-handle-request workspace/configuration): Use
eglot--workspace-configuration-plist.
(eglot-workspace-configuration): Document variable.

GitHub-reference: per https://github.com/joaotavora/eglot/issues/590
GitHub-reference: per https://github.com/joaotavora/eglot/issues/790
GitHub-reference: per https://github.com/joaotavora/eglot/issues/1033