Make search model configurable on server

- Expose ability to modify search model via Django admin interface
- Previously the bi_encoder and cross_encoder models to use were set
  in code
- Now it's user configurable but with a default config generated by
  default

src/database/models/__init__.py

@@ -102,6 +102,18 @@ class LocalPlaintextConfig(BaseModel):
     user = models.ForeignKey(KhojUser, on_delete=models.CASCADE)
 
 
+class SearchModel(BaseModel):
+    class ModelType(models.TextChoices):
+        TEXT = "text"
+
+    name = models.CharField(max_length=200, default="default")
+    model_type = models.CharField(max_length=200, choices=ModelType.choices, default=ModelType.TEXT)
+    bi_encoder = models.CharField(max_length=200, default="thenlper/gte-small")
+    cross_encoder = models.CharField(
+        max_length=200, default="cross-encoder/ms-marco-MiniLM-L-6-v2", null=True, blank=True
+    )
+
+
 class OpenAIProcessorConversationConfig(BaseModel):
     api_key = models.CharField(max_length=200)