Split statuses into required and warning/advisory

[shonfeder]

Currently, and for quite some time to come, opam repo maintainers have to exercise a lot of discretion in which CI failures they accept when merging packages. Eventually, we would like this to be fully automated, but in the meantime we can at least make the process easier via a more useful partitioning of the signals we send to github.

• CI results should broken out into two categories, warnings and errors.
• Results sent to github as statuses should be grouped accordingly.
• Once this is done, we can enable branch protection rules on the opam repo side.

This will prevent certain classes of errors we currently encounter from time to time, such as ocaml/opam-repository#26046 (comment)

Required checks should include:

• Reliable, binary linting checks
• The package building on supported platforms