sr/json: validate against metaschema with jsoncons

rapidjson does not support validating schemas above draft 5, so
previously we have been rewriting newer schemas in draft 4's metaschema
in order to validate them. This is error prone and gets increasingly
complicated for later drafts.

Instead, this commit pulls in the jsoncons library which has support for
validing schemas in drafts 4, 6, 7, 2019-09, 2020-12, which covers our
expected use cases.