validate mapping with json schema

[hauner]

avoid annoying failures caused by an error in the mapping.yaml by validating it with the json schema.

https://github.com/networknt/json-schema-validator looks good, it is using jackson and avoids having another json/yaml parser dependency.

[hauner]

.. this should also help to keep the schema up to date

[hauner]

it doesn't work as expected, the validator does not fully understand the schema (look like allOf does not work) and reports false errors.

IntelliJ doesn't complain and it validates a mapping, so the schema itself seems to be correct.