Validate instance types for GCP

[blakerosenthal]

Reference Issues or PRs

Closes #2718

Adds back the instances(region:str) function to the GCP provider and validates available instances against the nebari config.

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

How to test this PR?

Check out and install this branch, then follow the instructions to deploy nebari on GCP. Run nebari validate -c nebari-config.yaml to validate available instance types.

Any other comments?

[blakerosenthal]

@marcelovilla It looks like I'm missing a GOOGLE_CREDENTIALS environment variable in the tests. I spent some time with the test code trying to understand this but there's a lot of things that are instantiated at runtime. Is it obvious to you what's failing here?

https://github.com/nebari-dev/nebari/actions/runs/10948510295/job/30399687802?pr=2730

[marcelovilla]

@blakerosenthal I'll take a look at this tomorrow! Thanks for working on this 🚀