Make manual macro annotation unnecessary for simple aggregates, integrating Boost PFR

Hi, @pdimov ! Let me introduce you a nice thing that I contribute for a long time.
The [Boost PFR](https://github.com/boostorg/pfr) library provides everithing we need to get rid of `BOOST_DESCRIBE_STRUCT` macro(including extraction of fields names) and might be used in a lot of libraries that already relies of Boost Describe - Boost MySQL, Boost Json, etc.
It seemed nice to integrate PFR as yet another reflection library near the Describe into all of these libraries, but wait don't we have a more shorter way? Why don't we integrate PFR into Describe, keeping the old interface but without macro? And then all of these Describe-relied libraries will automatically get Boost PFR support for user defined types.

Of course we should bear in mind that PFR have limitations: https://www.boost.org/doc/libs/master/doc/html/boost_pfr/limitations_and_configuration.html

What do you think about such development? Does it worth to start working on it? I believe it's possible, but does it fit Describe's conception?
Please let me know if you are interested and don't hesitate to ask any question about Boost PFR.