Support `lsp-execute-command'

`lsp-execute-command' now works as an extension point again, but a deprecation
warning is shown if it is used to handle a command.

To implement this, leverage `cl-no-applicable-method': call
`lsp-execute-command' first, and then, if there is no implementation, go trough
the handler hash tables as usual.

lsp-mode.el

@@ -1088,6 +1088,9 @@ calling `remove-overlays'.")
 
 (defvar-local lsp--virtual-buffer-point-max nil)
 
+(cl-defgeneric lsp-execute-command (server command arguments)
+  "Ask SERVER to execute COMMAND with ARGUMENTS.")
+
 (defun lsp-elt (sequence n)
   "Return Nth element of SEQUENCE or nil if N is out of range."
   (cond
@@ -5168,9 +5171,18 @@ It will show up only if current point has signature help."
 
 (lsp-defun lsp--execute-command ((action &as &Command :command :arguments?))
   "Parse and execute a code ACTION represented as a Command LSP type."
-  (-if-let* ((action-handler (lsp--find-action-handler command)))
-      (funcall action-handler action)
-    (lsp--send-execute-command command arguments?)))
+  (let ((server-id (->> (lsp-workspaces)
+                        (cl-first)
+                        (or lsp--cur-workspace)
+                        (lsp--workspace-client)
+                        (lsp--client-server-id))))
+    (condition-case nil
+        (prog1 (lsp-execute-command server-id (intern command) arguments?)
+          (lsp--warn "`lsp-execute-command' is deprecated"))
+      (cl-no-applicable-method
+       (if-let ((action-handler (lsp--find-action-handler command)))
+           (funcall action-handler action)
+         (lsp--send-execute-command command arguments?))))))
 
 (lsp-defun lsp-execute-code-action ((action &as &CodeAction :command? :edit?))
   "Execute code action ACTION.