What does this PR do?

Add a possible implementation of using unkey's multiratelimiting. (https://www.unkey.com/docs/api-reference/v2/ratelimit/apply-multiple-rate-limit-checks)

To application developers this just allows specifying multiple checks in the checkRateLimitAndThrowError function:

await checkRateLimitAndThrowError([
      {
        rateLimitingType: "common",
        identifier: piiHasher.hash(req.nextUrl.pathname),
      },
      {
        rateLimitingType: "common",
        identifier: piiHasher.hash(`${req.nextUrl.pathname}-${requestorIp}`),
        opts: {
          limit: 5000,
        },
      },
    ]);

Under the hood this replaces the @unkey/ratelimit sdk with the underlying @unkey/api sdk while preserving the same behavior around timeouts and fallbacks.

It's not really finished and can be cleaned up some more, but I wanted to get some feedback on the direction first before spending time on that.
For API keys I left the original code for now, cause it's irrelevant to show what I have in mind.

I also don't care if you close this cause you want to do it differently :)

I'll leave some more comments throughout this PR.

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

It should just work as before. The check in the middleware now does 2 checks, one for path and one for path+ip

Checklist