Add (guix ipfs).

Add (guix ipfs).

This module allows for communicating with the IPFS
gateway over the HTTP interface.  The commit has been
cherry-picked from <https://issues.guix.gnu.org/33899>.

The procedures for adding and restoring file trees have
been removed as according to a reply issue 33899, a different
format will be used.  The procedure 'add-data' has been
exported as it will be used in the system test for IPFS.

* guix/ipfs.scm: New file.
* Makefile.am (MODULES): Add it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>