Add method to get current server time in milliseconds in ModuleApi (#18144)

- Add `get_current_time_msec()` method to the [module
API](https://matrix-org.github.io/synapse/latest/modules/writing_a_module.html)
for sound time comparisons with Synapse.
- Fixes #18104 

Signed-off-by: Ahmed Qashlan <ahmedelqashlan@gmail.com>
Co-authored-by: Eric Eastwood <madlittlemods@gmail.com>
Co-authored-by: Erik Johnston <erikj@jki.re>

changelog.d/18144.feature

@@ -0,0 +1 @@
+Add `get_current_time_msec()` method to the [module API](https://matrix-org.github.io/synapse/latest/modules/writing_a_module.html) for sound time comparisons with Synapse.

synapse/module_api/__init__.py

@@ -1846,6 +1846,10 @@ class ModuleApi:
             deactivation=deactivation,
         )
 
+    def get_current_time_msec(self) -> int:
+        """Returns the current server time in milliseconds."""
+        return self._clock.time_msec()
+
 
 class PublicRoomListManager:
     """Contains methods for adding to, removing from and querying whether a room