Enable conformance class configuration

[moradology]

This PR adds a conformance_classes parameter to the LandingPageMixin and all clients downstream from BaseCoreClient or AsyncCoreClient. This feature was added in a prior commit but appears to have been lost in the shuffle - a test has been added to hopefully avoid the regression going forward.

Closes #169

[philvarner]

I just ran this against the STAC API Validator and is passed the conformance part of the tests (though it blows up because the /collections endpoint is wrong)

[geospatial-jeff]

LGTM! There will be some merge conflicts due to #199

[rsmith013]

@moradology In the filter extension PR, I added a method list_conformance_classes to generate the list of conformance classes based on which extensions were enabled by the server. I think it would be good to move this PR in that direction rather than adding an extra instance attribute to the core client which needs to be configured. Thoughts?

[rsmith013]

nice!

[moradology]

@rsmith013 I like the idea of separating base conformance classes from those which are clearly extensions but am not entirely sold on not making the base classes configurable (I could be convinced but wonder how many not-fully-compliant STACs are out there). I've added the behavior from your PR though, as it looks like a win

[lossyrob]

What is the merge strategy here with #165? Should this be updated and merged, and then #165 updated with these changes, or do the changes not touch each other?

[rsmith013]

@lossyrob I would merge this first, then I expect there would be a minor conflict with #165 but should be easy enough to resolve.

[lossyrob]

Sounds good - @moradology can you update to latest and then I'll merge?

[moradology]

@lossyrob Pinging you so that you can push the button. Should be good to go