DEVX-7907: Update Verify2 Silent Authentication workflow options

[superchilled]

This PR updates the available options for a Verify v2 Silent Authentication workflow. Specifically it:

• Updates Verify2::Channels::SilentAuth:
  • Sets redirect_url and sandbox ivars in #initialize
  • Defines custom setter methods for the two new ivars (with input validation)
  • Defines getter methods for the two new ivars (via attr_reader)
• Adds/updates tests for the above implemention
• Some additional refactoring/ improvements for other channels

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1da0593) 97.94% compared to head (9adeea8) 97.95%.

:exclamation: Current head 9adeea8 differs from pull request most recent head d3ffd54. Consider uploading reports for the commit d3ffd54 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #300 +/- ## ======================================= Coverage 97.94% 97.95% ======================================= Files 108 108 Lines 1999 2008 +9 ======================================= + Hits 1958 1967 +9 Misses 41 41 ```

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