feat: Add state.NoOpClient (#1047)

Adds a `state.NoOpClient` that can be used by plugins when `plugin.SyncOptions.BackendOptions` is nil:

```
	var stateClient state.Client
	if options.BackendOptions == nil {
		c.logger.Info().Msg("No backend options provided, using no state backend")
		stateClient = state.NoOpClient{}
	} else {
 		...
```

Author: hermanschaaf

state/state.go

@@ -28,3 +28,20 @@ func NewClient(ctx context.Context, conn *grpc.ClientConn, tableName string) (Cl
 	}
 	return nil, fmt.Errorf("please upgrade your state backend plugin. state supporting version 3 plugin has %v", versions.Versions)
 }
+
+type NoOpClient struct{}
+
+func (*NoOpClient) SetKey(_ context.Context, _ string, _ string) error {
+	return nil
+}
+
+func (*NoOpClient) GetKey(_ context.Context, _ string) (string, error) {
+	return "", nil
+}
+
+func (*NoOpClient) Flush(_ context.Context) error {
+	return nil
+}
+
+// static check
+var _ Client = (*NoOpClient)(nil)