Fix type in code signature

[mjwolf]

Change the type of code_signature.flags to keyword, which is what it should be. Also add a unit test that will verify all types are valid.

• Have you signed the contributor license agreement?  ✅
• Have you followed the contributor guidelines? ✅
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? n/a
• If submitting code/script changes, have you verified all tests pass locally using make test? ✅
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? ✅
• Is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed. ✅
• Have you added an entry to the CHANGELOG.next.md? n/a

[github-actions]

Documentation changes preview: https://ecs_bk_2382.docs-preview.app.elstc.co/diff

[andrewkroh]

👍