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/constant.go

@@ -1,9 +1,5 @@
 package gemini
 
-const (
-	GeminiVisionMaxImageNum = 16
-)
-
 var ModelList = []string{
 	// stable version
 	"gemini-1.5-pro", "gemini-1.5-flash", "gemini-1.5-flash-8b",