change: refactor Filter Extension conformance classes

[philvarner]

Related Issue(s): #158

Proposed Changes:

Refactors Filter Extension conformance classes to more closely align with OGC API Part 3 changes coming soon.

The current draft Simple CQL class is definitely going away (opengeospatial/ogcapi-features#579 (comment)), but I don't think the proposed new ones meet the needs of STAC API either 🤷🏻 I think we're going to need to keep our own that don't align with the OAF ones, but hopefully they'll align eventually. Since Part 3 is a work in progress, having our own conformance classes will help insulate us from those changes and allow us to move forward without waiting for those to be finalized.

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[m-mohr]

I feel like we should provide a mapping between conformance classes from OGC API and STAC API, as we do with the other conformance classes, too. Not providing any OGC CQL conformance classes will break compatibility between OGC tooling and STAC tooling completely. With the mapping implementors could more easily express what they support for both worlds. That may lead to a subset of comliancy only as you'll only add conformance classes from OGC that you fully support, but overall at least some compliance could be achieved.

[cholmes]

Overall looks good. I'd say the one thing to add somewhere is an overview of alignment / discussion w/ OGC. I saw that you decided to go a bit of a different route with keeping the basic spatial / time ones. It'd be good just give an overview of where we stand and where OGC stands and where/how we see them coming together. This could just be in a ticket or something, and linked to from this page, or on this page.