Suggestion: Make ObjectMapper open

Greeting,

May I suggest we make ObjectMapper open? This way, we can create mappers for our games using inheritance:

```
class MyGameMapper: ObjectMapper() {
init {
registerType(....)
// .....
}
}
```

Thanks for the consideration!

Search the repository for the ObjectMapper definition and inspect its current declaration and usages. Confirm that the requested inheritance example can work without changing existing mapper behavior, then run the relevant project tests to verify the change.

Written by the indexing model from the issue text.