core: Legacy SDK providers opt out of our new apply result check

The shim layer for the legacy SDK type system is not precise enough to
guarantee it will produce identical results between plan and apply. In
particular, values that are null during plan will often become zero-valued
during apply.

To avoid breaking those existing providers while still allowing us to
introduce this check in the future, we'll introduce a rather-hacky new
flag that allows the legacy SDK to signal that it is the legacy SDK and
thus disable the check.

Once we start phasing out the legacy SDK in favor of one that natively
understands our new type system, we can stop setting this flag and thus
get the additional safety of this check without breaking any
previously-released providers.

No other SDK is permitted to set this flag, and we will remove it if we
ever introduce protocol version 6 in future, assuming that any provider
supporting that protocol will always produce consistent results.

helper/plugin/grpc_provider.go

@@ -791,6 +791,15 @@ func (s *GRPCProviderServer) ApplyResourceChange(_ context.Context, req *proto.A
 		return resp, nil
 	}
 	resp.Private = meta
+
+	// This is a signal to Terraform Core that we're doing the best we can to
+	// shim the legacy type system of the SDK onto the Terraform type system
+	// but we need it to cut us some slack. This setting should not be taken
+	// forward to any new SDK implementations, since setting it prevents us
+	// from catching certain classes of provider bug that can lead to
+	// confusing downstream errors.
+	resp.LegacyTypeSystem = true
+
 	return resp, nil
 }