[video_player] Add html 5 video poster support (thumbnail) as a VideoPlayerWebOptions

[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.

Fixes flutter/flutter#166232

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[stuartmorgan-g]

Where is the combination PR for initial review that this was split from (per the documented process)?

[Ortes]

Sorry for the mess, I messed up while trying to rebase, it is clean now

[stuartmorgan-g]

It looks like this never got a test at the web implementation package level, which was missed during review.

packages/packages/video_player/video_player_web/example/integration_test/video_player_test.dart

Line 412 in ca9b946

group('allowRemotePlayback', () {

is an example of a test similar to what we should have for this.

[Ortes]

It looks like this never got a test at the web implementation package level, which was missed during review.

packages/packages/video_player/video_player_web/example/integration_test/video_player_test.dart

Line 412 in ca9b946

group('allowRemotePlayback', () {

is an example of a test similar to what we should have for this.

I added tests, they are AI generated but looks good to me and they pass of course. I hope that will do it

[stuartmorgan-g]

It's fine to use AI as part of your development process, but you are responsible for ensuring the quality of the code you submit for review regardless of how it was initially created.

[stuartmorgan-g]

LGTM