speed up VektonnBaseModel.json() for Vector model (see #3)

src/vektonn/dtos.py

@@ -36,11 +36,26 @@ class Config:
         json_dumps = orjson_dumps
 
 
+class ToDictMixin:
+    def dict(
+        self,
+        *,
+        by_alias: bool = True,
+        exclude_none: bool = True,
+        **kwargs,
+    ) -> dict:
+        return {
+            self.__fields__[field_name].alias if by_alias else field_name: value
+            for field_name, value in self
+            if value is not None or not exclude_none
+        }
+
+
 class ErrorResult(VektonnBaseModel):
     error_messages: List[str]
 
 
-class Vector(VektonnBaseModel):
+class Vector(ToDictMixin, VektonnBaseModel):
     is_sparse: bool
     coordinates: List[float]
     coordinate_indices: Optional[List[int]]