BE: Add reason and dismissal_types column in policy_dismissal table

We have introduced security_policy_dismissals table in :

!202431 (merged)

We need to add the following columns in the table:

• reason
• dismissal_types

Check the design to determine column type and constrains #510650[Multi-policies_v2__apply_dismissal.png]

We also need to update security_findings_uuids column to accept null values, since Any MR Request type MR approval policy can have violations without associated finding uuids.

Steps to validate

• policy_dismissals table is updated with the new columns - reason, dismissal_types
• security_findings_uuids column is optional