[Lazarus] JSON-Schema added

[Michael Van Canneyt]

Hi,

I added a JSON-Schema class to FPC. It currently supports the latest JSON-Schema draft (2020-12)
There also is a validator class. There is a testsuite, but testsuites only go so far, 
so I'd appreciate some feedback from people who have need for JSON-schema.
(in particular, the $ref handling is a bit hairy. Available documentation is
definitely lacking in clarity)

The next step is adding support for OpenApi (AKA swagger).

Michael.