Align STAC Transactions with OGC API - Features Transactions

[cholmes]

STAC Transactions extension pre-dated the OGC API work on transactions. But they are very similar and the OGC API has a growing community around it, so we should just align STAC with those. See http://docs.opengeospatial.org/DRAFTS/20-002.html and https://github.com/opengeospatial/ogcapi-features/tree/master/extensions/transactions/simple

This was discussed on the STAC call 7/20/20, and agreed to be the best coarse of action.