docs(event-handler): new custom serializer option

Author: heitorlessa

docs/core/event_handler/api_gateway.md

@@ -730,6 +730,49 @@ This will enable full tracebacks errors in the response, print request and respo
 		return app.resolve(event, context)
 	```
 
+### Custom serializer
+
+You can instruct API Gateway handler to use a custom serializer to best suit your needs, for example take into account Enums when serializing.
+
+=== "custom_serializer.py"
+	```python hl_lines="19-20 24"
+	import json
+	from enum import Enum
+	from json import JSONEncoder
+	from typing import Dict
+
+	class CustomEncoder(JSONEncoder):
+		"""Your customer json encoder"""
+		def default(self, obj):
+			if isinstance(obj, Enum):
+				return obj.value
+			try:
+				iterable = iter(obj)
+			except TypeError:
+				pass
+			else:
+				return sorted(iterable)
+			return JSONEncoder.default(self, obj)
+
+	def custom_serializer(obj) -> str:
+		"""Your custom serializer function ApiGatewayResolver will use"""
+		return json.dumps(obj, cls=CustomEncoder)
+
+	# Assigning your custom serializer
+	app = ApiGatewayResolver(serializer=custom_serializer)
+
+	class Color(Enum):
+		RED = 1
+		BLUE = 2
+
+	@app.get("/colors")
+	def get_color() -> Dict:
+		return {
+			# Color.RED will be serialized to 1 as expected now
+			"color": Color.RED,
+			"variations": {"light", "dark"},
+		}
+	```
 
 ## Testing your code