fix support for custom formatters (#1997)

[tomkralidis]

Overview

This PR updates formatters support to allow for custom formatters for item types and EDR.

Related Issue / discussion

#1997

Additional information

Dependency policy (RFC2)

• I have ensured that this PR meets RFC2 requirements

Updates to public demo

• I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines

[webb-ben]

Looks good to me. I will test later today. Some small questions / comments.

• This implements custom formatters for OAFeat and OARec GeoJSON responses. Can we add some additional documentation to clarify this? Is support for EDR CovJSON Formatters an entirely different issue? I have a simple Point-based CovJSON -> CSV formatter that I would like to synchronize these changes with.

• After this is merged I will migrate our OGR Formatter to https://github.com/cgs-earth/pygeoapi-plugins. I will want to figure out how much of this can be offloaded to the configuration options introduced. Depending on the path this takes, a general OGR formatter could subsume #2136 and our custom formatter.

[webb-ben]

Does the formatter need to response both f= and the Accept Header?