Flexible output formats for coverage providers

[barbuz]

Overview

Related Issue / discussion

#1821

Additional information

As described in the linked issue, this PR allows coverage providers to specify the output formats they can serve. The xarray provider has been adapted to provide both zarr and netcdf outputs regardless of the format of the source data; more formats can be easily added in the future to this, rasterio, or any new coverage provider.

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

[barbuz]

@tomkralidis I have opened this separate PR as you suggested in a comment to my xarray PR. Do you have opinions/suggestions about this, or do you know who could be a good reviewer?