[video_player] Added `onScrubbingCallback` to `VideoProgressIndicator`

[pedrolemoz]

This pull requests adds onScrubbingCallback to VideoProgressIndicator widget, as a nullable callback function.

Related issue: https://github.com/flutter/flutter/issues/144777

Pre-launch Checklist

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• [x] 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.)
• [x] I signed the [CLA].
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [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 this PR is [exempt from version changes].
• [x] I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style].
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is [test-exempt].
• [x] All existing and new tests are passing.

[tarrinneal]

@stuartmorgan for second review

[stuartmorgan-g]

Please see my comments in the issue about better understanding the use case. If this actually needs to be done at the level of the UI element, I'm not sure this is a sufficiently common use case to warrant being added to the package (vs. the application in question just implementing a custom seek bar).

[stuartmorgan-g]

Marking as a draft pending resolution of the high-level goal/design discussion in the issue.

[stuartmorgan-g]

Closing since the issue was auto-closed due to lack of update. If you decide to revisit this in the future, please feel free to file a new issue for discussion!