feat: support vertex imagen3

relay/adaptor/vertexai/imagen/model.go

@@ -0,0 +1,23 @@
+package imagen
+
+type CreateImageRequest struct {
+	Instances  []createImageInstance `json:"instances" binding:"required,min=1"`
+	Parameters createImageParameters `json:"parameters" binding:"required"`
+}
+
+type createImageInstance struct {
+	Prompt string `json:"prompt"`
+}
+
+type createImageParameters struct {
+	SampleCount int `json:"sample_count" binding:"required,min=1"`
+}
+
+type CreateImageResponse struct {
+	Predictions []createImageResponsePrediction `json:"predictions"`
+}
+
+type createImageResponsePrediction struct {
+	MimeType           string `json:"mimeType"`
+	BytesBase64Encoded string `json:"bytesBase64Encoded"`
+}