feat: add content property to Video control

[bl1nch]

Description

This PR adds a content property to the Video control. It allows developers to replace the standard media_kit playback controls with a custom Flet control (e.g., a Container, Stack, or Row with custom buttons/text/other controls).

Test Code

# Test code for the review of this PR
import flet as ft
import flet_video as ftv


async def main(page: ft.Page):
    video = ft.Ref[ftv.Video]()

    async def on_inner_btn_click():
        await video.current.stop()

    async def on_play_btn_click():
        await video.current.play()

    async def on_stop_btn_click():
        await video.current.stop()

    page.add(
        ftv.Video(
            playlist=[ftv.VideoMedia("https://user-images.githubusercontent.com/28951144/229373720-14d69157-1a56-4a78-a2f4-d7a134d7c3e9.mp4")],
            height=500,
            width=700,
            ref=video,
            content=ft.Stack(
                controls=[
                    ft.Button(content="Stop <EXAMPLE>", on_click=on_inner_btn_click),
                ]
            )
        ),
        ft.Button(content="Play", on_click=on_play_btn_click),
        ft.Button(content="Stop", on_click=on_stop_btn_click),
    )


ft.run(main=main)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• I signed the CLA.
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing tests pass locally with my changes
• I have made corresponding changes to the documentation (if applicable)

Screenshots

Additional details

Summary by Sourcery

Add support for customizing the Video control’s playback UI via a new content property that can replace the default controls.

New Features:

• Introduce a content property on the Video control to host a custom Flet control as the video’s UI overlay.
• Wire the Video control’s content property into the Flutter video widget so custom content can be used instead of the built-in media controls when enabled.

[sourcery-ai]

We've reviewed this pull request using the Sourcery rules engine