Feature request: Verify stac_extensions contains link to self

[l0b0]

Is your feature request related to a problem? Please describe.

Now that we've settled on adding self-references in the schemas we should verify this. It can't really be verified using a non-example, so we should write unit tests for each of the extensions.

Describe the solution you'd like

A test for each extension which verifies that a stac_extensions entry with the URL to the relevant extension and version (https://linz.github.io/stac/_STAC_VERSION_/[extension name]/schema.json) is present.

Describe alternatives you've considered

A non-example file won't work, because stac-node-validator only validates against the schemas listed in stac_extensions.