Adding the ability to call a local executable as an ACL Source

[jhbigler]

(I should have opened this issue before submitting my PR but I didn't. My mistake).

It would be nice if we could write our own executable to generate the YAML/CSV ACL for the security manager to enforce. That way, if someone wanted to use a source that is not covered by one of the other source types, they can simply write their own script to generate the ACL without having to write Scala/rebuild the security manager. For example, a script that queries a SQL database or an Elasticsearch index, or generates an ACL from a directory of YAML/CSV files (instead of a single file), etc.