Generate text and images using different providers

[xiaoWangSec]

Hi,

since some providers may not have drawing capabilities, could we add an IMG-PROVIDERS parameter to specify the drawing model.

This way, users won't need to switch providers when they want to draw after setting up the provider's model.

# in main.go, line63
provider = flag.String("provider", os.Getenv("AI_PROVIDER"), "Choose which provider to use")
imgProvider = flag.String("imgprovider", os.Getenv("IMG_PROVIDER"), "Choose which provider to generate image")

# in helper.go, line 872
func generateImg(prompt string, imgProvider string) {	
	if imgProvider == "pollinations" || imgProvider == "" {
		generateImagePollinations(prompt)
	}  else if imgProvider == "blackboxai" {
		generateImageBlackbox(prompt)
	} else {
		fmt.Fprintln(os.Stderr, "Such a provider doesn't exist")
	}
}