feature: support setting nomad-pack plan exit codes via environment

[angrycub]

Issue

Currently, you can override the exit codes for the nomad-pack plan command using flags; however, they are on the cumbersome side of things and quite a lot to add to each command invocation.

Suggested solutions

I'd like to solicit feedback on how people might like to have this implemented. Please use emoji reactions on this comment to vote for your preference of the two.

🎉 : - An environment variable per setting:

NOMAD_PACK_PLAN_EXIT_NO_CHANGES=0`
NOMAD_PACK_PLAN_EXIT_MAKES_CHANGES=1`
NOMAD_PACK_PLAN_EXIT_ERROR=255`

🚀 : - An environment variable that contains the combined values as a little JSON object:

NOMAD_PACK_PLAN_EXIT_CODES='{"no_changes":0, "makes_changes":1, "error":255 }'

Personally leaning toward 🚀 :, but please use the corresponding emoji reactions if you feel strongly one way or the other.
If this feature is a priority, please use a 👍 reaction on it.