Include some jsonschema documents in the pystac distribution

[gadomski]

When doing some testing for #949, I was reminded that every single validate call hits the network. This makes it pretty hard to use PySTAC in a network-denied environment. I don't see any reason why we couldn't bundle the most common schemas in the PySTAC distribution and load them from the local system for validation -- the currently-supported version (v1.0.0 as of today) of the main data structures, plus GeoJSON Feature, seem like obvious candidates. It would additionally prevent issues like #622, where validation broke because the GeoSON Feature schema was a 404.