Adds the ability to deny certain trigger subjects for functions

[autodidaddict]

1. Always rejects a certain set of subject spaces for functions: $SYS.>, $JS.>, and $NEX.>.
2. You can add more trigger subject restrictions in the node config using the deny_trigger_subjects field

This is what it looks like to try and devrun a function with a denied trigger subject:

❯ ./nex devrun ../examples/v8/echofunction/src/echofunction.js --trigger_subject='$NEX.events.>' --type=v8
Reusing existing issuer account key: /home/kevin/.nex/issuer.nk
Reusing existing publisher xkey: /home/kevin/.nex/publisher.xk
[ERR] 2024-06-18 14:21:56 - failed to devrun workload err=The trigger subject $NEX.events.> overlaps with subject(s) in this node's deny list

[kthomas]

Simple enough-- one comment about the deny_trigger_subjects being logged in what feels like an unexpected place. Otherwise LGTM.