Stac v1 validation fixes

[dthiex]

closes https://github.com/Open-EO/openeo-sentinelhub-python-driver/issues/84

Within this MR:

• checked that STAC jsons use v1.0.0. (version is hard coded to 1.0.0)
• stac.hbs
  • added experimental flag support
  • added support for license identifier as string
• added experimental flag to experimental collections (list was created by Dorothy who has the best overview here)
• fixed a few things stac validator reveled (all non experimental collections where checked)
  • hard coding the self link rootURL is not the best approach but for now okay