Support RFC 9421 Signatures

[evanp]

This is a tracking issue for support for RFC 9421 HTTP Message Signatures in Bridgy Fed. For full support, we need:

[ ] Accept RFC 9421 Signatures for GET requests (actors, objects, collections, ...)
[ ] Accept RFC 9421 Signatures for POST requests (inbox, sharedInbox)
[ ] Send RFC 9421 Signatures for GET requests (double-knocking, capabilities discovery, ...)
[ ] Send RFC 9421 Signatures for POST requests (double-knocking, capabilities discovery, ...)

Feel free break this up into more issues. This is a big task!