Driver interface `AddConstraint` improvement

overview

At the moment, AddConstraint's definition (docs) states:

        // AddConstraint adds a Constraint to Driver for a particular Template. Future
	// calls to Query may reference the added Constraint. Replaces the existing
	// Constraint if it already exists.
        AddConstraint(ctx context.Context, constraint *unstructured.Unstructured) error

However, if no Template has already been added, via AddTemplate, the behavior is undefined. At present, the rego driver "fails silently":

https://github.com/open-policy-agent/frameworks/blob/3f237e2710faee505189afd605cfe75ed89dd446/constraint/pkg/client/drivers/rego/driver.go#L150-L164

solutions

1. Define the behavior and enforce it. Update the docs and if the targets is not found err out with some helpful error like TemplateNotFound

targets, found := ...
if not found, error

2. Modify the signature of AddConstraint to also include the Template that we want to associate the Constraint with. This approach would probably require some rethinking or regrokking of the flow for both AddTemplate and AddConstraint so it may actually be more work than value.

Start with the AddConstraint implementation in constraint/pkg/client/drivers/rego/driver.go at the linked lines, then trace how AddTemplate associates templates and constraints. Resolve whether a missing Template should produce a helpful error or require a signature change, and update the interface documentation and behavior so the undefined case is explicit.