Generic support for API-side validation via validate_only param or similar

[melinath]

What kind of contribution is this issue about?

MMv1-based resource

Details

AIP-163 gives guidelines for how APIs can provide a mechanism to validate requests. Most APIs do not implement this AIP. For APIs that do implement it (or something similar), I believe we could add support for calling it during the plan computation process (since it looks like we have access to configured clients from CustomizeDiff).

At a minimum, it would be good to choose three resources that support validate_only and implement & document something that works for all of them. It could be a flag in MMv1, or it could be a shared helper function - maybe even a CustomizeDiff function if there were a way to make a generic one that just gets plugged in.

References

#20641

[rileykarson]

I'm not sure if many APIs have supported this, and I'm unsure if they'd support unknown values, which would be important for proper support on our end.

[rileykarson]

note: size/m to implement on our side, but API-side implementation that we can hook into generically is size/xxxxxl or something.

[melinath]

A lack of support for unknown values might be acceptable, since I think we could automatically ignore validation errors for those fields on the Terraform side.