Add support for described classes

[pdimov]

Basically add https://www.boost.org/doc/libs/1_77_0/libs/describe/doc/html/describe.html#example_to_json and https://www.boost.org/doc/libs/1_77_0/libs/describe/doc/html/describe.html#example_from_json to the library proper so that described structs and classes work automagically, with no additional user effort.

An open issue is how to treat base classes; in the first iteration, we may only enable support for types with no base classes (as reported by describe_bases), which will allow us to postpone the decision without breaking code.