Decompose HttpClient and serialization method about log4net.elasticsearch

[jptoto]

@michal-duda It wasn't originally developed to be inherited which is why the coupling is pretty tight on that one. I think it's great if that code helped you but I'm not sure I see the need to decouple it for the sake of this particular code. If there is a reason to modify the json appender or enable multple deseralizers in the future I will look into doing this.

[michal-duda]

@jptoto I understand that it was meant to be used like that. Anyway I think would be nice to have way how to use let say JSON.net instead of default serializer.

[jptoto]

@michal-duda I'll take a look at doing this tonight. If it seems practical, I can add it in. I'm intentionally using the custom serializer to keep the library from having any dependencies other than log4net.