 Franklin support of STAC Layer extension endpoints

This task is to implementa Franklin STAC API Layer extension endpoints according to the [STAC API Layer extension spec](https://github.com/azavea/stac4s/tree/master/docs/stac-api-spec/extensions/layer)

This issue is a place holder, the actual work is happening here: https://github.com/azavea/franklin/issues/599