Share private Terraform providers across spaces (parity with module space_shares)

Workaround

None, the only two alternatives are not possible for supporting our use case: inherit_entities = true on the entity container so it can see root-owned providers. That also inherits every other root entity (stacks, contexts, policies, worker pools) and grants humans Space Read on root so we lose the isolation per space we need. public = true, which publishes the plugin to other Spacelift accounts after login, we cannot share these providers publicly.

Problem

We cannot keep spaces isolation and still consume one private provider from shared catalog modules. Module registry already has the sharing model we need; provider registry does not.