Block certain subjects from function trigger subjects

[autodidaddict]

Proposed change

Define a set of subjects/subject prefixes that are never allowed to be used by function triggers, and then possibly allow this to be supplemented by additional restrictions in the node configuration file.

Use case

We don't want functions to be able to listen on things like $SYS.> or $JS.> as that would potentially cause fatal problems depending on where the trigger function subscriptions are. Operators may also wish to further block additional prefixes/subjects from being used by function triggers.

Contribution

No response

[autodidaddict]

This is covered by PR #285