Autosave `confidential` in Issuable form

What does this MR do and why?

• Add input checkbox support in autosave
• Autosave confidential in Issuable form

Screenshots or screen recordings

How to set up and validate locally

1. Open a new issue : http://127.0.0.1:3000/flightjs/Flight/-/issues/new
2. Select 'This issue is confidential and should only be visible to team members with at least Reporter access.'
3. change type to 'Incident'

Checkbox status is persistent

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #357045