Added option to select serializer

[hartmark]

Fixes #420

• Changed so we always import JSON.net and added conditional just for System.Text.Json on newer dotnets.
• Opened up the JsonSerializerFactory so we can set another serializer if wanted.

I wanted to create a simple test for checking that the correct class was used as default.
I however noted that all tests returned it was using SystemTextSerializer even on the older dotnets.
See the Serializer_Should_Use_Correct_Default test.

[hartmark]

It was indeed using the JSON.net serializer now with latest push in the net40 unittest project.

[hartmark]

@abatishchev Any status for this PR?

[abatishchev]

Sorry to make you waiting! Will come back to it tonight, if not then shortly.