Plugin architecture for job dependency module

[SteVwonder]

In previous ☕ hours, we thought it would be cool to have a job dependency module in flux-core that supported plugins from flux-core as well as other projects (e.g., flux-depend) or from vendors. The analog here would be to jobtap. The depend module could handle registering the appropriate services, parsing RPC payloads, extracting the depend section from jobspec, and then passing the relevant info to registered/discovered plugins.

The depend module could then be responsible for waiting for a "thumbs up" from every plugin for each job. Once all of the plugins have signed off, the job can move out of the DEPEND state.

In RFC26, we have the concept of scheme (e.g., string or fluid). Assuming that scheme gets extended to include things like event or file, I thought it might be cool for plugins to register with the module what scheme they support. Then if a job comes in with a dependency scheme that no plugin is registered for, the dependency module can raise an exception and reject the job.

If people like the general idea, then I figure we can iterate on the exact idea/protocol in this discussion thread.