fix: Make cluster's project id non-computed attribute and add constant-ness check

[maciaszczykm]

Fixes PROD-2308 PROD-2328.

Closes #43 as it covers the same issue.

[linear]

PROD-2308 add constant-ness validator to terraform provider

PROD-2328 terrraform provider bug still present

[michaeljguarino]

So with this pr we'd never backfill the project data from the apis state I'm guessing?

[michaeljguarino]

I think this looks correct. One thing i'm wondering is what would happen for clusters that had the project id slurped into state from the prior versions but it isn't explicitly set? I suppose you can workaround it by just adding the project id explicitly?

[maciaszczykm]

So with this pr we'd never backfill the project data from the apis state I'm guessing?

Right, that's one of the flaws.

I think this looks correct. One thing i'm wondering is what would happen for clusters that had the project id slurped into state from the prior versions but it isn't explicitly set? I suppose you can workaround it by just adding the project id explicitly?

Yep, it should fix the issue if it will appear.