opam admin: more structured output

[avsm]

Our BuildKite CI setup is now regularly running various opam admin commands on the archive and pushing the results to a logs repository to the lints branch.

The intention is to make a lightweight CLI tool (obi) that checks on these and alerts package maintainers about any linting issues in their packages. For this, it would be useful if the opam admin commands output in a more structured format (for example JSON).

The commands currently running are opam admin lint and opam admin cache (to check for bad archives).

[avsm]

We are now running opam admin check in there as well, and pushing the results, so adding that to the potential structured output queue :)