Make name, maintainer, contact_email, url, description, version, and license optional

What does this MR do?

Makes a number of fields that are only needed for nitrile package and nitrile publish optional. This is especially useful for applications that are not published to the registry.

Changes to public APIs

See above. Users may remove information from their nitrile.yml if they want.

Author's checklist (required)

• Intermediate commits compile (use git rebase -i main if not)
• Newly added code has a style consistent with the rest of the repository
• Newly added code is documented (guidelines)
• If bugs have been solved, tests have been added (guidelines)
• A changelog entry has been added (guidelines)

Related issues