Add inlayHint capability.

Servers might check it, e.g. metals.

Author: kurnevsky

lsp-mode.el

@@ -3828,7 +3828,8 @@ disappearing, unset all the variables related to it."
                       (diagnostic . ((dynamicRegistration . :json-false)
                                      (relatedDocumentSupport . :json-false)))
                       (linkedEditingRange . ((dynamicRegistration . t)))
-                      (inlineCompletion . ())))
+                      (inlineCompletion . ())
+                      ,@(when lsp-inlay-hint-enable '((inlayHint . ((dynamicRegistration . :json-false)))))))
      (window . ((workDoneProgress . t)
                 (showDocument . ((support . t))))))
    custom-capabilities))