Add collection field to default list of fields to include in output

[robintw]

Related Issue(s): #
Fixes #257

Description:
The STAC spec states that Items should have a collection attribute, which should give the ID of the collection the item belongs to (see here. stac-fastapi was not returning this collection attribute in its output for Items, which was causing various downstream tools (such as stacterm to fail. This PR changes the list of default fields to include in the output to include the collection field.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.