Fixup codec based on real live STAC catalog

[jisantuc]

Overview

Some fields here were required, even though pystac doesn't think it needs to write them. It's not important who's right from the perspective of having something demoable tomorrow, so I'm making them optional as I discover that they're missing. For the non-required strings it's probably correct that they're not there. For the non-required lists, it's a bit of a nuisance that the spec doesn't just say they're required, so we wouldn't have to deal with the empty list vs null semantics for the thousandth time. 🤷‍♂️

Also there's a weird STAC thing where bboxes are just bbox: [a, b, c, d] on stac items, but are {"spatial": {"bbox": [ [a, b, c, d]]}} in collections, so I had to mess with encoding / decoding / case classes a bit.

Checklist

• Description of PR is in an appropriate section of the CHANGELOG and grouped with similar changes if possible

Testing Instructions

• serdespec

[notthatbreezy]

Looks good, we should definitely debrief after the sprint and figure out what our priorities are with the library to help us help our future selves more.