annotations support

[crazy-max]

fixes #332
fixes #355
needs #353
closes #351

alternative to #351 that sets an output and a bake definition for annotations: https://github.com/moby/buildkit/blob/master/docs/annotations.md

[aerosol]

Hey guys, this is causing invalid json carried over in steps metadata, an example that just broke our build pipeline:

(...) ,"annotations":["manifest:org.opencontainers.image.created=2023-11-30T12:46:22.687Z",manifest:org.opencontainers.image.description=Simple, open source, lightweight (< 1 KB) and privacy-friendly web analytics alternative to Google Analytics.,manifest:org.opencontainers.image.licenses=AGPL-3.0,manifest:org.opencontainers.image.revision=7bf1e2a6ed54b88e475898a18f9690bc5f690de4,manifest:org.opencontainers.image.source=https://github.com/plausible/analytics,manifest:org.opencontainers.image.title=analytics,manifest:org.opencontainers.image.url=https://github.com/plausible/analytics,"manifest:org.opencontainers.image.version=r2023.11.30-265"]}

Clearly there's some JSON encoding missing there. The second annotations item isn't surrounded with quotes.