Similar to Realm Java we should expose the option of writing copies of Realm files. This enables multiple features

• Make it easy to  create backups
• Encrypt a non-encrypted Realm
• Create asset files that can be bundled in Android APK's

// No proper filesystem support in Kotlin stdlib, use Strings or expose okio Paths?
Realm.writeCopyTo(destination: String)
Realm.writeEncryptedCopyTo(destination: String, key: ByteArray)

Note, that in order to support new Android 11 security mechanisms on external storage, we might want to expose a variant on Android that returns an outputbuffer. See https://github.com/realm/realm-java/issues/6974