feat: Add GEMINI_VISION_MAX_IMAGE_NUM configuration

- Introduced `GEMINI_VISION_MAX_IMAGE_NUM` to README files for better user guidance.
- Updated `env.go` to retrieve the maximum image number from environment variables, defaulting to 16.
- Modified image handling logic in `relay-gemini.go` to respect the new configuration, allowing disabling of the limit by setting it to -1.
- Removed hardcoded constant for maximum image number in `constant.go` to streamline configuration management.

relay/channel/gemini/relay-gemini.go

@@ -108,9 +108,10 @@ func CovertGemini2OpenAI(textRequest dto.GeneralOpenAIRequest) *GeminiChatReques
 				})
 			} else if part.Type == dto.ContentTypeImageURL {
 				imageNum += 1
-				//if imageNum > GeminiVisionMaxImageNum {
-				//	continue
-				//}
+
+				if constant.GeminiVisionMaxImageNum != -1 && imageNum > constant.GeminiVisionMaxImageNum {
+					continue
+				}
 				// 判断是否是url
 				if strings.HasPrefix(part.ImageUrl.(dto.MessageImageUrl).Url, "http") {
 					// 是url，获取图片的类型和base64编码的数据