implement SkRateLimiterService with Leaky Bucket rate limiting

packages/backend/src/core/TimeService.ts

@@ -0,0 +1,27 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { Injectable } from '@nestjs/common';
+
+/**
+ * Provides abstractions to access the current time.
+ * Exists for unit testing purposes, so that tests can "simulate" any given time for consistency.
+ */
+@Injectable()
+export class TimeService {
+	/**
+	 * Returns Date.now()
+	 */
+	public get now() {
+		return Date.now();
+	}
+
+	/**
+	 * Returns a new Date instance.
+	 */
+	public get date() {
+		return new Date();
+	}
+}