Expose supported api extensions

[geospatial-jeff]

STAC Items and Collections have the stac_extensions field which tells the client what extensions are supported. This is missing from the API spec which makes it difficult to programmatically determine which extensions are supported by a particular API implementation. This is particularly important for clients who are interacting with multiple API implementations which don't necessarily use the same STAC extensions.

I think it would make sense to add an optional stac_api_extensions field to either the landing page or capabilities document which links to the extensions used by the API.

[m-mohr]

Duplicate of #27 or at least similar ;-)

[jisantuc]

If we're calling this a duplicate, #27 should be retitled to include the stac_* fields intent -- I don't think it's obvious what that issue contains, which makes detecting overlap when opening new issues hard

[m-mohr]

Good call, I renamed #27.