feat: command flag to override parallel pool size

[itays333]

what

add -P / --parallel-pool-size flag to allow overriding pool size during plan/apply commands

why

Can be helpful in many scenarios IMO, but my main reason is that in some cases running multiple projects in parallel, which access the same resource (e.g via google_project_iam_member) can cause apply failures or sometimes state inconsistency. It should probably be addressed in the terraform provider, but i saw many cases of this in multiple resources. It would be good to have the option to apply serially with -P 1 from time to time

tests

• Added flag parsing test cases
• Tested manually e2e

[lukemassa]

It seems like this is more about toggling between running in parallel vs running in serial, does --parallel-plan work for this use case?

[itays333]

It seems like this is more about toggling between running in parallel vs running in serial, does --parallel-plan work for this use case?

--parallel-plan is a server-level configuration that requires redeploying Atlantis to modify. This PR provides more flexibility by:

• Enabling dynamic adjustment of the pool size for specific PR runs when serial execution is needed, without server redeployment
• Providing the ability to temporarily increase pool size beyond the default configuration. for example - useful when when spinning up resources that takes long time to provision over many projects