core: Use libgpgme to add GPG signatures to detached metadata for commit object

Add an optional dependency on gpgme to add GPG signatures into the
detached metadata, with the key "ostree.gpgsigs", as an "aay", an
array of signatures (treated as binary data).

The commit command gains a --gpg-sign=<key-id> argument.  Also add an
argument --gpg-homedir to set the GPG homedir where we look for
keyrings.