Added support of "startapp" deep links (#1648)

* Added "startapp" deep link support

* Remove link_type param, added create_startapp_link method

* Write tests for create_startapp_link method

* Refactor with black & isort

* Added CHANGELOG

aiogram/utils/deep_linking.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 __all__ = [
     "create_start_link",
     "create_startgroup_link",
+    "create_startapp_link",
     "create_deep_link",
     "create_telegram_link",
     "encode_payload",
@@ -77,9 +78,37 @@ async def create_startgroup_link(
     )
 
 
+async def create_startapp_link(
+    bot: Bot,
+    payload: str,
+    encode: bool = False,
+    encoder: Optional[Callable[[bytes], bytes]] = None,
+) -> str:
+    """
+    Create 'startapp' deep link with your payload.
+
+    If you need to encode payload or pass special characters -
+        set encode as True
+
+    :param bot: bot instance
+    :param payload: args passed with /start
+    :param encode: encode payload with base64url or custom encoder
+    :param encoder: custom encoder callable
+    :return: link
+    """
+    username = (await bot.me()).username
+    return create_deep_link(
+        username=cast(str, username),
+        link_type="startapp",
+        payload=payload,
+        encode=encode,
+        encoder=encoder,
+    )
+
+
 def create_deep_link(
     username: str,
-    link_type: Literal["start", "startgroup"],
+    link_type: Literal["start", "startgroup", "startapp"],
     payload: str,
     encode: bool = False,
     encoder: Optional[Callable[[bytes], bytes]] = None,