Add setting for enabling Continuous Vulnerability Scans

What does this MR do and why?

Update ProjectSecuritySetting with an attribute to control whether continuous vulnerability scans should run for the related project.

This is one of a series of MRs:

1. add attribute to the database and model - 👈 current MR
2. update the presenter to make the attribute available - !131315 (merged)
3. add mutation to toggle the attribute - !131450 (merged)

Screenshots or screen recordings

n/a

How to set up and validate locally

n/a

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 #424374 (closed)