bin/commit: Add --add-metadata that accepts g_variant_print() format

bin/commit: Add --add-metadata that accepts g_variant_print() format

Mostly adding this for use in test cases; it allows us to add e.g.
integers, and we need to deal with byteswapping those.

Someone mind also find it useful to add fully structured metadata, although most
of those users should be using a real language and not shell script.

Closes: #1372
Approved by: jlebon