Validate extensions from schema URLs

[jisantuc]

Overview

This PR introduces mechanisms to read extension validators from JSON schemata. This flow involves handling a few distinct kinds of error:

1. We might get a network error when reading the schema
2. There might not be valid json at the URL
3. The valid json might not be a valid JSON schema
4. Even if it's valid, we might not be able to resolve links in the JSON schema (circe-json-schema is only into local files because the Everit loader is only into local files)
5. The "uri" might not actually be a URI that sttp likes to make requests with despite sttp saying it's fine after parsing it

after all of this, we should eventually get a function using that Schema from Item => Item or Collection => Collection that we can use for validation on read.

This costly process is wrapped in a Ref (we're not on 3.x yet but the basics are the same) so that we don't have to try the same sequence of resolutions every time we see the special string.

Checklist

• New tests have been added or existing tests have been modified

Notes

I've asked in STAC gitter about what others are doing about the json schema links. This might end up being a roll-our-own situation if we don't have good library support for resolving with links. I think we can do that on top of circe-json-schema without too much hassle though.

Testing Instructions

• CI should be sufficient I think thanks to the validation spec

[pomadchin]

LGTM and works locally! I left a couple of question in comments though.