Allow setting -parallelism= flag

[drigz]

TL;DR

Setting terraform plan -parallelism=100 can give a ~3x speedup for large projects. I'd like to be able to set this with guardian plan, but I don't see an option for it: https://github.com/abcxyz/guardian/blob/main/cli.md#plan

Detailed design

This could either be an additional flag for guardian plan, or some generic way of adding additional terraform flags, such as `guardian plan -terraform-flags="-parallelism=100"`.

Alternatives considered

Just being slightly more patient :)

Additional information

Evidence for the speedup when using guardian run -- ...:

[verbanicm]

@drigz thanks for the suggestion, this seems like a good performance improvement.

What version of Guardian are you on?

[drigz]

Embarrassing question, apparently we're on 1.0.5... I'll take a look at whether we can update, are there any breaking changes from v1 to v2? The release doesn't have any notes unless I'm looking in the wrong place: https://github.com/abcxyz/guardian/releases/tag/v2.0.0

[verbanicm]

@drigz Yea there are, specifically flag changes. I dont believe we created a migration guide, but I would suggest running the templates again and viewing the diff.

https://github.com/abcxyz/guardian/blob/main/abc.templates/README.md

I only ask because I can add this to both but I am not sure we are going to maintain V1 going forward, but if this is too heavy of a lift, we can probably add it to both versions. Let me know if you need any help, just ping me.

[drigz]

A nice-to-have feature like this is definitely not something to bother backporting - but thank you for asking!