move jsonfg from plugin to default

**Is your feature request related to a problem? Please describe.**

I noticed a [plugin jsonfg](https://github.com/cgs-earth/pygeoapi-plugins/blob/master/pygeoapi_plugins/formatter/jsonfg.py), by now is it an idea to move it to default status, since in may it was adopted by OGC as formal standard

**Additional context**

which is the formal way to request jsonfg from a server?

accept: draft: application/vnd.ogc.fg+json / final: application/fg+json (although not registered at IANA yet)

do we support the shortcut ?f=fg or f=jsonfg