Trigger hooks when updating assignees via GraphQL

What does this MR do and why?

We stringify the options hash so we should be checking the options using a String key

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 #372038 (closed)