Implement optional count in Feature Providers

[webb-ben]

Overview

Implement count field in provider def allowing providers with expensive count actions to skip returning a numberMatched value in the normal feature collection response when count is false.

Related Issue / discussion

Closes: #1969
Pending: #2174

Additional information

• Providers that do not have this added either require a count or may return it without performing an expensive count query.
• https://github.com/cgs-earth/pygeoapi-plugins/blob/master/pygeoapi_plugins/provider/postgresql.py offers a Postgis provider with a psuedocount which is faster but less accurate than a full count.

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