Add mode of operation in Vulnerability policy

[sahibamittal]

Description

Add mode of operation for Vulnerability Policy.

Addressed Issue

https://github.com/DependencyTrack/hyades/issues/957

Checklist

• [x] I have read and understand the contributing guidelines
• [ ] This PR fixes a defect, and I have provided tests to verify that the fix is effective
• [x] This PR implements an enhancement, and I have provided tests to verify that it works as intended
• [x] This PR introduces changes to the database model, and I have updated the migration changelog accordingly
• [ ] This PR introduces new or alters existing behavior, and I have updated the documentation accordingly