Firebase Analytics: UserProperty value type

First of all thanks for this awesome plugin, I really love it! ❤️

It's currently not possible to collect user properties in the analytics module the way that Firebase allows it. This plugin only features string value (see: https://github.com/EddyVerbruggen/nativescript-plugin-firebase/issues/125 and https://github.com/EddyVerbruggen/nativescript-plugin-firebase/blob/6649521a043957fbde4b5280a90e513738b745b1/firebase.d.ts#L681), but in the Firebase console I can configure boolean and number as type as well.

I personally don't like the idea of converting my numbers to strings, because when Firebase stores them as string as well, I won't be able to filter by that values correctly (e.g. "greater than" comparisons) in the Firebase console.