[SDK-3753] Use single ObjectMapper

[jimmyjames]

ObjectMapper is thread-safe and should be reused. This PR adds a new class ObjectMapperProvider, which returns a singleton instance for internal usages to process JSON data.

[poovamraj]

Looks good to me. But I am wondering whether we should use this through an interface so that we can provide an option to use a custom serializer/deserializer?

Use of concrete class feels it could be hard to move things around if required in the future.

[jimmyjames]

Looks good to me. But I am wondering whether we should use this through an interface so that we can provide an option to use a custom serializer/deserializer?

Can you share an example of what you are thinking? The class is intended for internal usage only (can add a commit to clarify the javadocs), so if we need to add or change functionality in the future we can.

[poovamraj]

I was wondering something like where we use an interface

public class Auth0ObjectMapper {
       public String deserialize();

       public Object serialize();
}

So that we can provide it as an option if users want a custom instance to use like below

Auth0ObjectMapper = new Auth0ObjectMapper {
      private Gson gson = new Gson();

      ...
}
auth.useMapper(customMapper)

[jimmyjames]

Adding a hook for custom serialization/deserialization isn't something this PR aims to provide. It's not currently in our backlog but could certainly be added if it's something we want to consider for v2 or beyond.