Need better validation of various plugin's options in jobspec

[SteVwonder]

Right now we have three sets of plugins that look to the jobspec of jobs for their options:

• jobtap plugins
• job-shell plugins
• dependencies modules/plugins

We already have support for dependency modules to register which schemas they support. That way if a user submits a job with a dependency scheme with no corresponding module that supports the scheme, they get an error shortly after submission.

We don't have a similar setup for jobtap or job-shell plugins. As an example of why this is important, in #3802 a user submitted a job with the alloc-bypass option, but the alloc-bypass jobtap plugin was not loaded. This resulted in weird behavior (the job hung unexpectedly). It would be nice if the job could be rejected early on (either at ingestion or in the initial processing by jobtap plugins) rather than sitting in the scheduler queue. Same goes for a malformed job-shell plugin.

It would also be good to have validation for the opposite case (i.e., the user adds an option for a module/plugin that is loaded). As an example, maybe the user add --setattr system.data-staging=foo to their flux mini invocation. It would be great if the data-staging plugin could run a quick validation to see if foo is a valid value.

Since we have three sets of plugins/modules that need the same functionality, might be a good opportunity to use the same abstraction to solve the problems in a consistent way.