Enhance and refactor the Brave Ads component updater

[moritzhaller]

This ticket includes:

• Enable a/b testing different versions of a resource, e.g. text classifier.
• Refactor the Brave Ads component updater: Change name from "user model" to "ads resource" and move code into the brave ads component.
• Remove bat-native-usermodel dependency from ads.

[moritzhaller]

cc @dimmu