core: Add detached metadata, readd metadata to commits

Previously I thought we'd have to ditch the current commit
format to avoid a{sv} due to

See https://bugzilla.gnome.org/show_bug.cgi?id=673012

But I realized that we don't really have to care about
unpacking/repacking commit objects, so let's just re-expose the
existing metadata a{sv} in commits in the API.

Also, add support for "detached" metadata that can be updated at any
time post-commit.  This is specifically designed for GPG signatures.

https://bugzilla.gnome.org/show_bug.cgi?id=707379