Don't generate paging links if provider created them

[mikemahoney218-usgs]

Overview

This PR changes itemtypes.py to allow providers to generate their own 'next' links, which then disable the limit-offset paging built into pygeoapi. As discussed in #1981, we want to move away from limit-offset paging for the data we're serving from Postgres and move towards keyset paging instead, and I believe this change would let us handle that from within our provider.

We just check for a next link in the response before disabling the generation of both prev and next links, as the Features standard mentions that prev links may be skipped based on implementation. As such, if the provider has generated a next link but not a prev link, we assume that was intentional.

Related Issue / discussion

#1981

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