Create `User` and `Breadcrumb` data classes from a Map

[kahest]

see https://github.com/getsentry/team-mobile/issues/59

Often Hybrid SDKs have to create data classes out of a Map, See:

https://github.com/getsentry/sentry-dart/blob/b8b9b7b2608c35e4a3f7340ae9d2f0e9c06d984f/flutter/android/src/main/kotlin/io/sentry/flutter/SentryFlutterPlugin.kt#L272-L286

Android and iOS expose extension methods for this, such as, use it in RN