See https://github.com/realm/realm-kotlin/issues/406

Add proxy support options to the AppConfiguration similar to Realm Java.

This includes:

• `AppConfiguration.Builder.authorizationHeader()`
• `AppConfiguration.Builder.customRequestHeader(header, value)`
• `AppConfiguration.Builder.customRequestHeaders(headers: Map<String, String>)`
• Extending the Network Transport to support it. Right now this is done in Java as ObjectStore doesn't have support for custom headers (if I remember it correctly).