Add rate limiting functionality to Flask app

- Introduced MemoryRateLimiter for managing request limits.
- Added configuration options for enabling rate limiting and setting request limits and time windows.
- Implemented methods to enforce rate limits and build rate limit keys based on request context.
- Integrated rate limiting checks into the request handling process.

tests/test_rate_limit.py

@@ -0,0 +1,37 @@
+import flask
+
+
+def test_rate_limit_disabled_by_default():
+    app = flask.Flask(__name__)
+
+    @app.route("/")
+    def index():
+        return "ok"
+
+    client = app.test_client()
+
+    for _ in range(3):
+        rv = client.get("/")
+        assert rv.status_code == 200
+
+
+def test_rate_limit_blocks_after_threshold():
+    app = flask.Flask(__name__)
+    app.config.update(
+        RATELIMIT_ENABLED=True,
+        RATELIMIT_REQUESTS=2,
+        RATELIMIT_WINDOW=60,
+    )
+
+    @app.route("/")
+    def index():
+        return "ok"
+
+    client = app.test_client()
+
+    assert client.get("/").status_code == 200
+    assert client.get("/").status_code == 200
+
+    rv = client.get("/")
+    assert rv.status_code == 429
+