feat: provider group

[alambare]

in EODAG library:

• add a by_group option on core.available_providers. If set, we return the unique list of groups instead of the list of provider names.
• update list_product_types to accept group name as provider names. Calling list_product_types with a group name set in the provider name will return the union of product types from all the providers of this group.

in EODAG server:

• use group when set instead of provider name

group is an optional field

Example:

I have 2 provider configuration in EODAG:

• earth_search
• earth_search_cog

I configure both with group earth_search.

• [x] When I list the product types for provider earth_search, EODAG return the merged list of product types from earth_search and earth_search_cog.

• [x] In server mode, the returned product types display earth_search as provider name.

• [x] When I search for products with provider earth_search, EDOAG search on both provider using the existing fallback mechanism. The returned products display earth_search as provider name.

• [x] When I ask for queryables with provider earth_search the response is the intersect of queryables from earth_search and earth_search_cog.

• [x] available_providers returns only one entry for both: earth_search.