better description of federation backends

[alambare]

• title should be different from id. We should probably introduce a title property in EODAG providers

• add eodag:priority to advertise the priority give to each provider

• in collections, federation:backends should not be set in summaries but instead, directly at the root level of the collection. Similarly to bands in STAC.

• with database in EODAG, replace enabled=True/False by status=offline/online and ensure it is present in the Provider object so it can be displayed as well in the landing page.

• sortables in STAC?

• queryables currently return the union of all properties of federated backends. this is probably not the best idea as users can make combination of parameters that are invalid.