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.en.md

@@ -81,6 +81,7 @@ You can add custom models gpt-4-gizmo-* in channels. These are third-party model
 - `UPDATE_TASK`: Update async tasks (Midjourney, Suno), default `true`
 - `GEMINI_MODEL_MAP`: Specify Gemini model versions (v1/v1beta), format: "model:version", comma-separated
 - `COHERE_SAFETY_SETTING`: Cohere model [safety settings](https://docs.cohere.com/docs/safety-modes#overview), options: `NONE`, `CONTEXTUAL`, `STRICT`, default `NONE`
+- `GEMINI_VISION_MAX_IMAGE_NUM`: Gemini model maximum image number, default `16`, set to `-1` to disable
 
 ## Deployment
 > [!TIP]