Provide JSON artifact with details for consumption by archweb

When releasing a new version of the releng build artifacts we require a JSON consumable, that can be picked up by the promotion tooling and/or archweb to add metadata about a new release.

The consumable needs to implement at least the following attributes from the respective archweb models for this to work.

Interesting metadata is:

• kernel version
• archiso version
• ipxe version