Add config plugin for RedirectScheme

[XChikuX]

Issue

Firstly, thank you for creating this awesome library. OAuth is a doozy to work with and I am very greatful for a library such as this.

However, I had to spend an embarrassing amount of time to get the RedirectScheme portion working.

// build.gradle
android {
  defaultConfig {
    manifestPlaceholders = [
      appAuthRedirectScheme: 'io.identityserver.demo.auth'
    ]
  }
}

Can we create an expo-config plugin to automate this? Without needed to mess with build.gradle or Android.manifest files ourslves?