[video_player] Add poster attribute for html video tag in video_player_platform_interface

[Ortes]

This PR adds support for the poster attribute on web by introducing a poster field in VideoPlayerWebOptions. It allows setting a thumbnail image for videos using the native HTML5 poster property, improving the out-of-the-box web experience.

This PR is is the changes to platform_interface for this one

Solve this issue: https://github.com/flutter/flutter/issues/166232

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• [x] I linked to at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under[^1].
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[stuartmorgan-g]

Marking as a draft since, per the documented process, the creation of this PR should happen only after the combined PR has been approved.

[Piinks]

Greetings from stale PR triage 👋, what is the status of this PR? Looking through it and the linked PRs we weren't sure.

[stuartmorgan-g]

@Ortes Is this updated to match the current state of https://github.com/flutter/packages/pull/8979 ?

[Ortes]

@Ortes Is this updated to match the current state of #8979 ?

Yes all fine,

Sorry for the mess I made while trying to rebase

[Ortes]

Thanks for the review @stuartmorgan-g hopefully it is all fixed