feat: scoped tenant variables

[mik-ky]

This PR adds support for Scoped Tenant Variables.

Project Tenant Variables

• environment_id becomes optional, but the variable must be scoped to an environment via the scope.
• Backwards compatible -- if server does not support this, it will call the existing V1 endpoint. If it does, it will migrate existing state and will use V2 endpoint.
• Import updated to allow v2 variables

Common Tenant Variables

• Adds scope block
• Backwards compatible -- same as Project Tenant Variables
• Import updated to allow v2 variables

Other changes

• (V1) Terraform state keeps ids as a composite key. In V2, we use TenantVariableId as the id as now there could be multiple environments scoped to a single variable.
• Adds an option to override feature flags in our test provider in tests
• Because of the Tenant Variable API, it can't run in parallel. A keyed mutex was added to ensure we don't run into concurrency issues

[sc-93478]

[mik-ky]

Not so much related to tenant variables, but a minor reusability change on a generic schema

[bec-callow-oct]

Looks good, and the issues we discussed are resolved