Add bake metadata JSON output

[jnewland]

Description

Making the JSON printed in the "Bake definition" portion of this Action's output available as an step output would facilitate the use of this information in Actions job summaries. Practically, I'm hoping to use this information to explain the context dependency tree present in a bake manifest to contributors so that they can understand how their change may impact other downstream images.

[reardonm]

This issue looks complete in v6 that has a metadata output, but it would be helpful if it was available as a file, in addition to or rather than an environment variable. I find that a large multi-stage build with provenance enabled it gets quite large, and I have to first write it to a file to do much with it.

[danielhollas]

I find that a large multi-stage build with provenance enabled it gets quite large, and I have to first write it to a file to do much with it.

Indeed, I have ran into the same issue, reported in #239