item level derived_from link

[NorahBrown]

The current ItemSerializer.stac_to_db does not write any non inferred links to the links field in the items table.

We propose :
An adjustment to filter for derived_from in the input and then explicitly set the links value

# Proposed mod to stac_fastapi/sqlalchemy/serializers.ItemSerializer.py

from stac_fastapi.types.links import filter_links
# ...
class ItemSerializer(Serialiazer):
# ...
    @classmethod
    def stac_to_db(...):
        # ...
        # Before instantiation of the database.Item, pull out derived_from link
        derived_from = None
        # Remove inferred link
        temp_links = filter_links(stac_data['links'])
        # Assign derived_from link
        for temp_link in temp_links:
            if temp_link['rel'] == 'derived_from':
                derived_from = [temp_link]
                break
        # ...
        return database.Item(
            # ...
            links=derived_from,
            # ...
            )

We would be happy to provide a PR.
Thanks