Allow verification token when creation external audit event destination

What does this MR do and why?

When the a streaming audit event destination is created, allow the user to optionally pass in their own value for verificationToken to be used. If the user provides the parameter in the API call, use that value instead of generating one.

How to set up and validate locally

1. Create destination using https://docs.gitlab.com/ee/administration/audit_event_streaming.html#use-the-api also pass verification token.
2. Get list of destinations using https://docs.gitlab.com/ee/administration/audit_event_streaming.html#use-the-api-1 and verify verification token.
3. Create another destination, this time don't pass verification token.
4. Get list of destinations using https://docs.gitlab.com/ee/administration/audit_event_streaming.html#use-the-api-1 and verify verification token is not empty.

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 #360813 (closed)