Add support for JSON request format instead of XML

[jachym]

Description

Like @tomkralidis is writing:

Another option is to extend PyWPS to support an 'outputformat'
parameter like 'application/json' which would simply transform the XML
response into JSON.  Or to be closer to the WPS spec allow one to
define/send process input which allows takes over the entire response
into JSON.

Having said this, we do this for pycsw as very simple step just before
the response is sent.  Examples:

Example response:
https://github.com/geopython/pycsw/blob/master/tests/expected/suites_default_post_GetRecordById-json.xml
Code: https://github.com/geopython/pycsw/blob/master/pycsw/core/formats/fmt_json.py#L35

[tomkralidis]

Given recent developments (WPS 2.0 REST-API), moving to a future release for once we implement same.