fdo-approve-users: move the ruby script into its own script

This way we can run this locally for testing as opposed to hoping we didn't introduce a syntax error 34 lines deep in a shell script inside a yaml section.

This is on top of !247 (merged)

cc @bentiss

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note: I'm not sure if the {{ readFile }} bit is needed (or works) because to get to that point one needs to understand helmfiles :) Might be that having it in the same directory is enough.