Add EO extension parsing

[notthatbreezy]

Improvement

The EO extension is helpful when describing multiband imagery and will make tile links more discoverable in the long-run.

Notes + Context

This should probably follow from #51 and once that is done this can follow

Any other thoughts @jisantuc ?

[pomadchin]

@notthatbreezy 💯 could be a nice demo to show how stac4s works with the Landsat8 STAC

[jisantuc]

I don't have other thoughts on this in particular, but I think it would be good to try out how the validation strategy discussed in azavea/franklin#147 works before we go too far down this path

[pomadchin]

I was getting familiar with stac4s and decided to create some tests with the API examples, and for tests I used some modified version of LC catalog; hopefully it would be useful as a demonstration of fields that are missing: https://github.com/azavea/stac4s/pull/73/files

Probably we're also interested in a view extension as well (sun angles are not in the eo extension anymore).