Update OPENNEM Exchange

[VIKTORVAV99]

Description

Update OPENNEM Exchange to V4 API. Se parent issue for more details.

[gudyo]

@VIKTORVAV99, would it be alright if I give this issue a go? I am interested in contributing to this project.

[VIKTORVAV99]

Yes of course!
I'll be reviewing #8587 today which does the same for production.

So if possible limit your changes to just the exchange code so we don't get merge conflicts we need to sort out later.

Thanks for contributing!