Implement typed header parsing for `ucan` header

[cdata]

After we integrate support for changes to rs-ucan landing in https://github.com/ucan-wg/rs-ucan/pull/28, we will have to contend with a new world where HTTP request headers that use UCAN-based JWT for authorization may also include auxilliary ucan headers (see: https://github.com/ucan-wg/ucan-as-bearer-token#2-request-headers).

We already benefit from typed header parsing for the authorization header. We should author an extension to the typed headers that are offered by axum to include something to make parsing ucan headers more convenient and consistent.