Continue refactoring itemtypes.py?

[mikemahoney218-usgs]

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

Some components in itemtypes.py (for instance, get_collection_items) are somewhat lengthy and could be factored into multiple functions. We're wondering if PRs to refactor these into more modular components would be welcome.

On our end, having smaller functions would make it easier to monkey-patch custom functionality on top of existing pygeoapi features going forward (for instance, #2060 )

[tomkralidis]

Given the API refactor a couple of years back, I'd like to keep this consistent for the time being. We can revisit once we have a stable release.