revert: reinstate stac_extensions schema definitions

[dwsilk]

ref: eb3d494 (commit being reverted)
discussion: #60 (comment)
upstream issue: stac-extensions/template#10

These appear to be useful for checking that our own examples are correct, and to remain consistent with all of the JSON Schemas in the STAC community extensions.

If they are removed upstream / determined to no longer be needed, then a new PR and discussion can be created.

[l0b0]

How do we test this? It can't be tested on the extensions themselves. We'd have to create a separate dummy extension for each extension, referencing the extension under test, then create a JSON document which references the dummy extension and verify that it fails validation.

Update: I've changed my mind. My initial thinking was that the STAC metadata file would tell the JSON validator which schema to validate against. But it's also possible for the JSON validator caller to tell it which schema to use, independently of the contents of the STAC file. Since the schema reference in this case is outside of the STAC file it makes sense to enforce the contents of the STAC file this way.

Unfortunately I don't think we can test this with the current stac-node-validator, because it only looks at the contents of the STAC files to determine which schemas to validate against.

[blacha]

Unfortunately I don't think we can test this with the current stac-node-validator, because it only looks at the contents of the STAC files to determine which schemas to validate against.

The unit tests @MitchellPaff wrote could be used to validate this.

[l0b0]

Unfortunately I don't think we can test this with the current stac-node-validator, because it only looks at the contents of the STAC files to determine which schemas to validate against.

The unit tests @MitchellPaff wrote could be used to validate this.

That's a good point! Created #79.