MVP AppConfiguration

[sync-by-unito]

Add minimum API for AppConfiguration

• AppId
• BaseUrl (required for testing)

class AppConfiguration {    val foo = bar
  class Builder {
    constructor(appId: String)
    fun baseUrl(url: String) // Or if better URL abstraction exits
  }
}

// usage (same as Java)
val app = App(AppConfiguration.Builder(appId).baseUrl(myUrl).build())

In Realm Java baseUrl accepted a String because URL throws a checked exception and that was too annoying to deal with for the safety it provided. We should probably re-evaluate if we can use URL or URI for Kotlin.