To achieve this signatures are now generated by a custom
Tesla Middleware placed after the FollowRedirects Middleware.
Any requests which should be signed needs
to pass the signing key via opts.
This also unifies the associated header logic between fetching and
publishing, notably resolving a divergence wrt the "host" header.
Relevant spec demands the host header shall include a port
identification if not using the protocols standard port.
Fixes: https://akkoma.dev/AkkomaGang/akkoma/issues/731
4c4982d611