Support "no command eval" style allow listing of (unsigned) commands

[zsims]

This is in addition to #14. The Buildkite UI still needs to be used to specify an "initial" command. This command will currently be unsigned.

This feature adds a "no command eval" style approach that any file in the repository can be executed unsigned provided the rules from #14 are true (e.g. no plugins are referenced, and the step doesn't have a signature)