Support using image generation models like Flux via Replicate

Enables using any image generation model on Replicate's Predictions
API endpoints.

The server admin just needs to add text-to-image model on the
server/admin panel in organization/model_name format and input their
Replicate API key with it

Create db migration (including merge)

src/khoj/database/models/__init__.py

@@ -280,6 +280,7 @@ class TextToImageModelConfig(BaseModel):
     class ModelType(models.TextChoices):
         OPENAI = "openai"
         STABILITYAI = "stability-ai"
+        REPLICATE = "replicate"
 
     model_name = models.CharField(max_length=200, default="dall-e-3")
     model_type = models.CharField(max_length=200, choices=ModelType.choices, default=ModelType.OPENAI)