Implement typed header parsing for ucan header

[blaine]

Summary

Problem

Currently, parsing the ucan header in axum is a fair amount of boilerplate work.

Impact

Everyone using UCANs for http-based authorization in e.g. axum need to implement the extractors by hand. This is both a high barrier to entry and a lot of work.

Solution

rs-ucan (or adjacent crate) should ship with an out-of-the-box extractor for axum.

Detail

We have a nascent implementation in fission-server, and will work on extracting that to be generally reusable.

Additional context

See also: subconsciousnetwork/noosphere#68