feat(sub2api): add host adapter client and tests

internal/host/sub2api/subscriptions.go

@@ -0,0 +1,11 @@
+package sub2api
+
+import "context"
+
+func (c *Client) AssignSubscription(ctx context.Context, req AssignSubscriptionRequest) (SubscriptionRef, error) {
+	var ref SubscriptionRef
+	if err := c.postJSON(ctx, "/api/v1/admin/subscriptions/assign", req, &ref); err != nil {
+		return SubscriptionRef{}, err
+	}
+	return ref, nil
+}