[feature] Support rate limiting through policy_effect extension

It would be great if Casbin could support rate limiting natively by extending the `policy_effect` expression and implementing a custom `Effector`.

The idea is to introduce a new `rate_limit` function in policy_effect like this:

```ini
[policy_effect]
e = rate_limit(10, minute, allow, sub)
```

This means "allow at most 10 requests per minute, only count allowed requests, and bucket by subject". The four parameters are:

- `max`: maximum request count within the time window
- `unit`: time window unit, could be `second`, `minute`, `hour`, or `day`
- `count_type`: either `allow` (only count allowed requests) or `all` (count all requests including denied ones, useful for preventing brute-force attacks)
- `bucket`: the field(s) to group by, such as `all`, `sub`, `obj`, `act`

A `RateLimitEffector` implementing the `Effector` interface would parse this expression, maintain internal bucket states (counters and window timestamps), and decide whether to allow or deny based on current count. The bucket key is generated from the request context according to the bucket type, for example `sub:alice` when bucket is `sub`.

To implement this, we need to extend the policy_effect expression parser to recognize the `rate_limit` function, create a new `RateLimitEffector` that implements the `Effector` interface with internal state management, and make sure the enforce context (sub, obj, act) is passed to the effector so it can generate proper bucket keys.