lsp-erlang: add config option for diagnostics on save

This can be set via customize

In future we should auto-gen the entire section based on the output of

    elp config

clients/lsp-erlang.el

@@ -55,6 +55,19 @@ It can use erlang-ls or erlang-language-platform (ELP)."
   :group 'lsp-erlang
   :package-version '(lsp-mode . "6.2"))
 
+;; ---------------------------------------------------------------------
+;; ELP config settings.  Eventually auto-gen these from the output of `elp config`
+
+(defcustom-lsp lsp-erlang-elp-diagnostics-on-save
+  nil
+  "Update native diagnostics only when the file is saved."
+  :group 'lsp-erlang-elp
+  :type 'boolean
+  :lsp-path "elp.diagnostics.onSave.enable")
+
+;; ---------------------------------------------------------------------
+
+
 ;; erlang-ls
 
 (defcustom lsp-erlang-ls-server-path