Add MSC3030 experimental client and federation API endpoints to get the closest event to a given timestamp (#9445)

MSC3030: https://github.com/matrix-org/matrix-doc/pull/3030

Client API endpoint. This will also go and fetch from the federation API endpoint if unable to find an event locally or we found an extremity with possibly a closer event we don't know about.
```
GET /_matrix/client/unstable/org.matrix.msc3030/rooms/<roomID>/timestamp_to_event?ts=<timestamp>&dir=<direction>
{
    "event_id": ...
    "origin_server_ts": ...
}
```

Federation API endpoint:
```
GET /_matrix/federation/unstable/org.matrix.msc3030/timestamp_to_event/<roomID>?ts=<timestamp>&dir=<direction>
{
    "event_id": ...
    "origin_server_ts": ...
}
```

Co-authored-by: Erik Johnston <erik@matrix.org>

synapse/handlers/federation.py

@@ -68,6 +68,37 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
+def get_domains_from_state(state: StateMap[EventBase]) -> List[Tuple[str, int]]:
+    """Get joined domains from state
+
+    Args:
+        state: State map from type/state key to event.
+
+    Returns:
+        Returns a list of servers with the lowest depth of their joins.
+            Sorted by lowest depth first.
+    """
+    joined_users = [
+        (state_key, int(event.depth))
+        for (e_type, state_key), event in state.items()
+        if e_type == EventTypes.Member and event.membership == Membership.JOIN
+    ]
+
+    joined_domains: Dict[str, int] = {}
+    for u, d in joined_users:
+        try:
+            dom = get_domain_from_id(u)
+            old_d = joined_domains.get(dom)
+            if old_d:
+                joined_domains[dom] = min(d, old_d)
+            else:
+                joined_domains[dom] = d
+        except Exception:
+            pass
+
+    return sorted(joined_domains.items(), key=lambda d: d[1])
+
+
 class FederationHandler:
     """Handles general incoming federation requests
 
@@ -268,36 +299,6 @@ class FederationHandler:
 
         curr_state = await self.state_handler.get_current_state(room_id)
 
-        def get_domains_from_state(state: StateMap[EventBase]) -> List[Tuple[str, int]]:
-            """Get joined domains from state
-
-            Args:
-                state: State map from type/state key to event.
-
-            Returns:
-                Returns a list of servers with the lowest depth of their joins.
-                 Sorted by lowest depth first.
-            """
-            joined_users = [
-                (state_key, int(event.depth))
-                for (e_type, state_key), event in state.items()
-                if e_type == EventTypes.Member and event.membership == Membership.JOIN
-            ]
-
-            joined_domains: Dict[str, int] = {}
-            for u, d in joined_users:
-                try:
-                    dom = get_domain_from_id(u)
-                    old_d = joined_domains.get(dom)
-                    if old_d:
-                        joined_domains[dom] = min(d, old_d)
-                    else:
-                        joined_domains[dom] = d
-                except Exception:
-                    pass
-
-            return sorted(joined_domains.items(), key=lambda d: d[1])
-
         curr_domains = get_domains_from_state(curr_state)
 
         likely_domains = [