buildx: history load .dockerbuild

[crazy-max]

• ~needs #340~

Add load func to load build records from a .dockerbuild OCI tarball. This will be used as follow-up to generate the summary instead of relying on the summary.json generated by export-build tool, so we can use standard types.

Output example: https://github.com/docker/actions-toolkit/actions/runs/9268352262/job/25496795839#step:3:1053