Add call_function_later to DAP

This adds a new call_function_later API to DAP.  This arranges to run
a function after the current request has completed.  This isn't used
yet, but will be at the end of this series.

Reviewed-by: Kévin Le Gouguec <legouguec@adacore.com>

gdb/python/lib/gdb/dap/server.py

@@ -281,6 +281,12 @@ class Server:
                         return
         self.send_event(event, body)
 
+    @in_dap_thread
+    def call_function_later(self, fn):
+        """Call FN later -- after the current request's response has been sent."""
+        with self.delayed_fns_lock:
+            self.delayed_fns.append(fn)
+
     # Note that this does not need to be run in any particular thread,
     # because it just creates an object and writes it to a thread-safe
     # queue.
@@ -321,6 +327,12 @@ def send_event_maybe_later(event, body=None):
     _server.send_event_maybe_later(event, body)
 
 
+def call_function_later(fn):
+    """Call FN later -- after the current request's response has been sent."""
+    global _server
+    _server.call_function_later(fn)
+
+
 # A helper decorator that checks whether the inferior is running.
 def _check_not_running(func):
     @functools.wraps(func)