fix: make context menu clicks cover the whole cell

[sissbruecker]

Description

Fixes the grid context menu to always provide the correct item information in the open event, as well as the dynamic content handler. Previously in both cases the item could be missing if the context click was not directly on the grid cell content element, but on the table cell element itself. The approach of using the sourceEvent to get the event context is taken from the Typescript example in the docs: https://vaadin.com/docs/latest/ds/components/grid/#context-menu

Fixes #2444

Type of change

• Bugfix
• Feature

[vaadin-bot]

Hi @sissbruecker and @sissbruecker, when i performed cherry-pick to this commit to 14.8, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick 2606328
error: could not apply 2606328... fix: make context menu clicks cover the whole cell (#2458)
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add ' or 'git rm '
hint: and commit the result with 'git commit'

[vaadin-bot]

This ticket/PR has been released with platform 23.0.0.alpha1 and is also targeting the upcoming stable 23.0.0 version.