[Iguana] Add non-functional iguana executable

Summary:
Add an iguana executable, but don't add any actual functionality to it, only an argument parser with help text and a version string.

Later, we can add functionality to have it actually run scripts.

Test Plan: ninja iguana && ninja check-iguana

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16771