Add ability to add optional redirect url

[SecondeJK]

This PR allows Verify v2 requests for Silent Auth to go out with the optional redirect_url

Description

The option modifies the VerificationWorkflow object to allow custom keys to be added (which are likely to be soon used in other Verify channels)

Motivation and Context

New parameter that is already available

How Has This Been Tested?

New test added to make sure that the outgoing response has the correct key

Example Output or Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[dragonmantank]

LGTM