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
The old code was unnecessarily complicated, full of unused and/or
duplicated functions making it hard to understand what will actually
happen and for whom at runtime.
Since we only support a single HTTP backend this can be greatly simplified.
Now everything gets default options from a single place and only
functions to modify parts actually difffering across calls are exposed.
