Extends storage.Persist to accept a Content-Type argument, which in
the GCS backend is persisted with the object to ensure that the object
is served back with this content-type.
This is not yet implemented for the filesystem backend, where the
parameter is simply ignored.
This should help in the case of clients which expect the returned
objects to have content-types set when, for example, fetching layers
by digest.
This gets rid of the package called "server" and instead moves
everything into the project root, such that Go actually builds us a
binary called `nixery`.
This is the first step towards factoring out CLI-based functionality
for Nixery.
