Make cluster a required parameter for sources, sinks, matviews, and indexes

[bobbyiliev]

Currently, the cluster parameter is not required and will use to the default cluster.

[benesch]

Moving a comment from Slack to here for discussion:

This will play nicely once ALTER SET CLUSTER for materialized views is in public preview, and I’ll have to think of a way to handle this for the rest of the objects.

ALTER SET CLUSTER, even for materialized views, had to be abandoned due to unforeseen complications. So we're not going to have ALTER SET CLUSTER support anytime soon.

It seems like that's okay, though? Can't we just set ForceNew on the cluster parameter?

I guess there's likely a question of how to do the migration. But it seems like it might be possible to do this with a state migration? Spitballing:

• Create a release of the Terraform provider that ensures that a cluster output parameter is always set with the cluster of the object, regardless of whether the user has provided the input parameter.
• Create a release of the Terraform provider that makes the cluster input parameter mandatory. Add a state migration to this release that fills in the input parameter based on the value of the output parameter. (Is this possible? Not sure).
• Instruct users to upgrade through the releases in sequence, and when they upgrade to the last version, to fill in cluster input parameters wherever they are missing. Thanks to the schema migration, this won't cause the resources to be replaced.