Implement rate limiting middleware in Freeway

[travis]

Implement a rate limiting middleware in Freeway. It should behave similarly to the prototype implementation in https://github.com/storacha/freeway/pull/109/files#diff-9a117d52a8a2cb3e5f498575bdfdea4dc32418180c09029454fbaf7f95ebe5a1R142 but should be supported by testing and documented.

For this iteration, we can rate limit if and only if the request does not have a valid "auth token."

Subtasks:

• [x] #139
• [x] #141