[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:
flutter/flutter#166232

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• 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 under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[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