This allows buildkitd daemon to define additional fields
that are added to all the provenance attestations that
BuildKit creates (by default from /etc/buildkit/provenance.d/).
These custom fields can provide additional context about
the environment BuildKit itself is running (eg. Github workflow)
and are not allowed to collide with the trusted fields created
by BuildKit itself.
Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>