lib: redirect handling by protocol handler

lib: redirect handling by protocol handler

Adds a `follow()` callback to protocol handlers, so they may decide how
to act on a `newurl` after a request has been done. This is optional.

This moves the HTTP code for handling redirects from multi.c to http.c
where it should be. If we ever add a protocol with its own logic, it
would install its own follow function.

Closes #16075