core: Add a function creating an archive-z2 content stream

core: Add a function creating an archive-z2 content stream

It is quite similar to the already existing
ostree_raw_file_to_content_stream function, so I factored the common
part to a separate function. The difference is that we cannot report
the size of the resulting stream.

Can be useful for serving a "bare" repository as a faked "archive-z2"
repository.

Closes: #308
Approved by: cgwalters