Switch to new JSON handling library

[OlegRa]

.NET Core 3.0 and .NET Standard 2.1 provides a new built-in set of classes for fast JSON handling in .NET - this is a new recommended way for handling JSON streams in case of forward-only processing. We can enable this support in a new major SDK release after implementing other .NET Core 3.0 related changes.

[OlegRa]

After analyzing all the pros and cons of this switch and required amount changes in the source code I've decided to postpone such critical change till next major release.

[OlegRa]

We have too many features in our deserialization code which heavily depends on JSON.NET library features and now I don't see any real reason for rewriting all this stuff using a new library. I'll not close it but now it has no fixed milestone. If someone will do it or provide real reason why should we implement it I can return back to this task.

[OlegRa]

We need this one for issue #144 (at least for the REST part of this issue).

[OlegRa]

Another good reason to postpone it till .NET 6 release: no-reflection solution via source generators!