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.

README.md

@@ -87,6 +87,7 @@
 - `UPDATE_TASK`：是否更新异步任务（Midjourney、Suno），默认为 `true`，关闭后将不会更新任务进度。
 - `GEMINI_MODEL_MAP`：Gemini模型指定版本(v1/v1beta)，使用“模型:版本”指定，","分隔，例如：-e GEMINI_MODEL_MAP="gemini-1.5-pro-latest:v1beta,gemini-1.5-pro-001:v1beta"，为空则使用默认配置(v1beta)
 - `COHERE_SAFETY_SETTING`：Cohere模型[安全设置](https://docs.cohere.com/docs/safety-modes#overview)，可选值为 `NONE`, `CONTEXTUAL`，`STRICT`，默认为 `NONE`。
+- `GEMINI_VISION_MAX_IMAGE_NUM`：Gemini模型最大图片数量，默认为 `16`，设置为 `-1` 则不限制。
 ## 部署
 > [!TIP]
 > 最新版Docker镜像：`calciumion/new-api:latest`