Add support for Anthropic models (#760)

* Add support for chatting with Anthropic's suite of models

- Had to use a custom class because there was enough nuance with how the anthropic SDK works that it would be better to simply separate out the logic. The extract questions flow needed modification of the system prompt in order to work as intended with the haiku model

src/khoj/database/models/__init__.py

@@ -84,6 +84,7 @@ class ChatModelOptions(BaseModel):
     class ModelType(models.TextChoices):
         OPENAI = "openai"
         OFFLINE = "offline"
+        ANTHROPIC = "anthropic"
 
     max_prompt_size = models.IntegerField(default=None, null=True, blank=True)
     tokenizer = models.CharField(max_length=200, default=None, null=True, blank=True)