Make UseNCronJobAsync() mandatory when startup jobs have been defined

[nulltoken]

Pull request description

Follow up to https://github.com/NCronJob-Dev/NCronJob/pull/121#discussion_r1826658953

Slight enhancement to control of startup jobs.

The lib will now throw on startup only when:

• Startup jobs have been defined
• UseNCronJobAsync hasn't been invoked

No change at the doc level were required as no wording was hinting to UseNCronJobAsync being optional.

This should prevent some misconfiguration at the user level.

PR meta checklist

• [ ] Pull request is targeted at main branch for code
• [ ] Pull request is linked to all related issues, if any.

Code PR specific checklist

• [ ] My code follows the code style of this project and AspNetCore coding guidelines.
• [ ] My change requires a change to the documentation.
  • [ ] I have updated the documentation accordingly.
• [ ] I have updated the appropriate sub section in the CHANGELOG.md.
• [ ] I have added, updated or removed tests to according to my changes.
  • [ ] All tests passed.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files with missing lines	Coverage Δ
src/NCronJob/NCronJobExtensions.cs	97.29% <ø> (ø)
src/NCronJob/Scheduler/QueueWorker.cs	70.58% <100.00%> (+2.54%)	:arrow_up: