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 ✅

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

❗ 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

@@           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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[SecondeJK]

possible changes, def not essential