Add support for Google Analytics 4

[botandrose]

Hello, thanks for rack-tracker! I've been using it for years, and so I really appreciate your efforts with it!

Google is pushing everyone onto GA4, so I want to add support for it to this gem. @ad-zaitsev already forked it and did an implementation, which seems to work just fine. I've simply copied it here in hopes to get it into an eventual gem release. I see that its lacking test and documentation, so I'm happy to put in the work to add those, if you are amenable to merging it afterwards!

Green light?

[timlawrenz]

@botandrose any chance to add specs to get the PR merged?

[botandrose]

If I get word that it would be merged afterwards, I'll put in the work!

[timlawrenz]

@bumi is that all that's needed for this to be merged?