Add config plugin for RedirectScheme

## 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?