Add platform to BrowserExtensionConnectedToServer event log

Created by: lguychard

Rel. #11447 (closed)

Last piece of the solution to track browser extension usage, for users who never click any injected links or take any code intelligence action, but visit the Sourcegraph instance with the browser extension connected.

I could have just added a browser argument parsed from the user agent string here, but figured standardizing on the platform sent by the browser extension like in other logs (#12533) or utm_sources (#12534) would be better for consistency.