[collections] add /collections/{collection_id} endpoints

[vincentsarago]

we already have /collections/{collection_id}/items/{item_id} and /mosaic/{search_id} endpoints but it would be nice to also have /collections/{collection_id} without needed registration

# Now
/mosaic/{search_id}. - Mosaic Endpoints 
/collections/{collection_id}/items/{item_id}` - Items Endpoint

# Proposed 

/mosaics or searches/{search_id}. - Mosaic Endpoints (based on search queries)
/collections/{collection_id}` - Collections Mosaic Endpoint
/collections/{collection_id}/items/{item_id}` - Items Endpoint