#1277 Add custom types for datetime in Telegram API

This commit adds a new file `custom_types.py` in the aiogram directory.
The inclusion of `DateTime` type ensures better compatibility with the Telegram Bot API. `DateTime` type is further enhanced to work with unixtime.

aiogram/custom_types.py

@@ -0,0 +1,17 @@
+import datetime
+from typing import Annotated
+
+from pydantic import PlainSerializer
+
+# Make datetime compatible with Telegram Bot API (unixtime)
+DateTime = Annotated[
+    datetime,
+    PlainSerializer(
+        func=lambda dt: int(dt.timestamp()),
+        return_type=int,
+        when_used="json-unless-none",
+    ),
+]
+
+# Make string compatible with custom lazy proxy
+# String = Union[str, LazyProxy]