search from STAC collection URL

[sbrunato]

Add a feature to allow searching items from a given STAC collection URL.

Use a mechanism similar as what is done in import_stac_items():

• Origin provider and download links will be set if collection comes from an EODAG server.
• If collection comes from a known EODAG provider, result will be registered to it, ready to download and its metadata normalized.
• If collection comes from an unknown provider, a generic STAC provider will be used.