Support user authentication extension

[simolus3]

Originally suggested by @rodydavis in https://github.com/simolus3/drift/discussions/2540.

Putting this on hold since, with the current build scripts, it's hard to create a customizable sqlite3 build for users. Once we're able to use Dart's native assets feature, we can allow users to opt-in to the extension and provide APIs for the exposed methods in this package.