Add /_synapse/admin/v1/event_reports endpoint (#3342)

Based on #3340 

This adds a `/_synapse/admin/v1/event_reports` endpoint, the same
Synapse has. This way existing tools also work with Dendrite.
Given this is already getting huge (even though many test lines),
splitting this into two PRs. (The next adds "getting one report" and
"deleting reports")

[skip ci]

clientapi/routing/admin.go

@@ -495,3 +495,45 @@ func AdminDownloadState(req *http.Request, device *api.Device, rsAPI roomserverA
 		JSON: struct{}{},
 	}
 }
+
+// GetEventReports returns reported events for a given user/room.
+func GetEventReports(
+	req *http.Request,
+	rsAPI roomserverAPI.ClientRoomserverAPI,
+	from, limit uint64,
+	backwards bool,
+	userID, roomID string,
+) util.JSONResponse {
+
+	eventReports, count, err := rsAPI.QueryAdminEventReports(req.Context(), from, limit, backwards, userID, roomID)
+	if err != nil {
+		logrus.WithError(err).Error("failed to query event reports")
+		return util.JSONResponse{
+			Code: http.StatusInternalServerError,
+			JSON: spec.InternalServerError{},
+		}
+	}
+
+	resp := map[string]any{
+		"event_reports": eventReports,
+		"total":         count,
+	}
+
+	// Add a next_token if there are still reports
+	if int64(from+limit) < count {
+		resp["next_token"] = int(from) + len(eventReports)
+	}
+
+	return util.JSONResponse{
+		Code: http.StatusOK,
+		JSON: resp,
+	}
+}
+
+func parseUint64OrDefault(input string, defaultValue uint64) uint64 {
+	v, err := strconv.ParseUint(input, 10, 64)
+	if err != nil {
+		return defaultValue
+	}
+	return v
+}