integration/hsic: add CreateOAuthClient via the v2 API client

Mints an admin API key and creates a keyType=client OAuth credential through gen/client/v2, exposed on ControlServer. Reusable by tests needing OAuth client credentials.

integration/control.go

@@ -1,6 +1,7 @@
 package integration
 
 import (
+	"context"
 	"net/netip"
 
 	clientv1 "github.com/juanfont/headscale/gen/client/v1"
@@ -22,6 +23,8 @@ type ControlServer interface {
 	ConnectToNetwork(network *dockertest.Network) error
 	GetHealthEndpoint() string
 	GetEndpoint() string
+	GetIPEndpoint() string
+	CreateOAuthClient(ctx context.Context, scopes, tags []string) (string, string, error)
 	WaitForRunning() error
 	Restart() error
 	CreateUser(user string) (*clientv1.User, error)