Only allow PNG icons in gallery and use commit link for pieces-azure-data-studio

[Charles-Gagnon]

VS Code requires PNG (since SVGs can contain code) so enforcing that for us as well.

Also updating the pieces-azure-data-studio extension to use a direct commit link n(follow up from #25173) to reduce the chance of changes to master breaking the gallery. I'm trying to think of a good way to enforce this as well and will follow up with adding a check for that if I come up with one - but for now we'll just ask people to update them as they submit new extensions.

FYI @caleb-at-pieces

[github-actions]

MSFT Gallery Maintainer Only
This is a reminder to update the Extensions Gallery list at https://github.com/Microsoft/azuredatastudio/wiki/List-of-Extensions once this PR has been merged in.
Instructions for doing this can be found here.

[caleb-at-pieces]

Thank you! @Charles-Gagnon