[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.

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

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

Is this ok like this ? here is the PR for platform_interface

[stuartmorgan-g]

From triage: @tarrinneal This should be ready for initial review.

[stuartmorgan-g]

~From triage: @Ortes apologies, we forgot to follow up with next steps here. Now that this has sign-off, the next step is to split out the platform interface component into a separate PR so that we can land it, as described at https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#changing-federated-plugins~

Never mind, I forgot https://github.com/flutter/packages/pull/8979 was already split out.

[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. https://github.com/flutter/packages/blob/ca9b946ca4e36da5ab5dbcf3d110571ceb671f48/packages/video_player/video_player_web/example/integration_test/video_player_test.dart#L412 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.

https://github.com/flutter/packages/blob/ca9b946ca4e36da5ab5dbcf3d110571ceb671f48/packages/video_player/video_player_web/example/integration_test/video_player_test.dart#L412

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