[ENH] - Validate instance types for Google Cloud

[marcelovilla]

Feature description

Right now, we're not validating whether instance types passed in the configuration are valid when initializing the infrastructure. However, we're doing it for AWS and DigitalOcean. See:

• https://github.com/nebari-dev/nebari/blob/aed2b926fc1176fa90e4b23e6ee9dc02ca0e861d/src/_nebari/stages/infrastructure/init.py#L517
• https://github.com/nebari-dev/nebari/blob/aed2b926fc1176fa90e4b23e6ee9dc02ca0e861d/src/_nebari/stages/infrastructure/init.py#L282

Value and/or benefit

Besides being consistent with what we have with other providers, it would enhance the user's experience when specifying unavailable/non-existing instance types before running into an error halfway through the deployment.

Anything else?

No response