feat(policies): allow custom builtin functions in Rego policies

[jiparis]

This PR adds extension capabilities to the Rego engine by using the builtins mechanism from OPA. Registering a new function is done through a call to builtins.Register with the builtin definition:

func RegisterHelloBuiltin() error {
	return Register(&ast.Builtin{
		Name: helloBuiltinName,
		Decl: types.NewFunction(
			types.Args(
				types.Named("name", types.S), // Digest to fetch
			),
			types.Named("response", types.A), // Response as object
		),
	}, getHelloImpl)
}

Implementation signature must be a topdown.BuiltinFunc. Check provided example and tests.

This call must be done before using the Rego engine, so ideally it should be done during package initialization (init()) or app initialization before instantiating Rego.

[jiparis]

to be able to devel CONTAINER_IMAGE policies.

[migmartri]

Cool, see my comments, thanks!