core(plugins): load plugins to validate artifacts

[patrickhulce]

Summary
Lighter version of #8374 that's all in one-go. The other breaking change we might want to consider before v5 related to plugins is that audit paths are currently relative to the master config instead of relative to the plugin's config, which might make more sense. i.e. in the plugin config you want to say 'use the audit right next to my config file'

Nearly all of these changes would be necessary to make that happen so I think it's pretty worthwhile :)

Related Issues/PRs
#8374

[patrickhulce]

Would like a decision here on what we consider breaking for v5. If the answer is "this is all puntable", that's fine :)

Two breaking changes discussed/blocked on this PR:

• Throwing on the use of non-blessed artifacts
• Changing what file audit paths in plugins are relative to

see

audit paths are currently relative to the master config instead of relative to the plugin's config, which might make more sense. i.e. in the plugin config you want to say 'use the audit right next to my config file'

[patrickhulce]

Friendly ping here.

By the lack of excitement I'm guessing we don't really care about the audit file path resolution behavior for v5, but explicit confirmation of this fact would be nice :)

[patrickhulce]

Closing in favor of #8374, this PR was only relevant as an easier transition path that could land before 5.0.0.