Add MediaGroupBuilder for media group construction (#1293)

Implemented a MediaGroupBuilder class in 'aiogram/utils/media_group.py' to help construct media groups. The class supports addition of different media types (audio, photo, video, document) to the media group with a maximum limit of 10 files. The functionality is demonstrated and usage is documented in 'docs/utils/media_group.rst'. Added related test cases in 'tests/test_utils/test_media_group.py'. This is to streamline and simplify the process of media group creation

docs/utils/index.rst

@@ -10,3 +10,4 @@ Utils
     web_app
     callback_answer
     formatting
+    media_group

docs/utils/media_group.rst

@@ -0,0 +1,46 @@
+===================
+Media group builder
+===================
+
+This module provides a builder for media groups, it can be used to build media groups
+for :class:`aiogram.types.input_media_photo.InputMediaPhoto`, :class:`aiogram.types.input_media_video.InputMediaVideo`,
+:class:`aiogram.types.input_media_document.InputMediaDocument` and :class:`aiogram.types.input_media_audio.InputMediaAudio`.
+
+.. warning::
+
+    :class:`aiogram.types.input_media_animation.InputMediaAnimation`
+    is not supported yet in the Bot API to send as media group.
+
+
+Usage
+=====
+
+.. code-block:: python
+
+    media_group = MediaGroupBuilder(caption="Media group caption")
+
+    # Add photo
+    media_group.add_photo(media="https://picsum.photos/200/300")
+    # Dynamically add photo with known type without using separate method
+    media_group.add(type="photo", media="https://picsum.photos/200/300")
+    # ... or video
+    media_group.add(type="video", media=FSInputFile("media/video.mp4"))
+
+
+To send media group use :meth:`aiogram.methods.send_media_group.SendMediaGroup` method,
+but when you use :class:`aiogram.utils.media_group.MediaGroupBuilder`
+you should pass ``media`` argument as ``media_group.build()``.
+
+If you specify ``caption`` in :class:`aiogram.utils.media_group.MediaGroupBuilder`
+it will be used as ``caption`` for first media in group.
+
+.. code-block:: python
+
+    await bot.send_media_group(chat_id=chat_id, media=media_group.build())
+
+
+References
+==========
+
+.. autoclass:: aiogram.utils.media_group.MediaGroupBuilder
+   :members: