fix: apply correct logic to consistency

[epbensimpson]

Fixes #30

• Adds AtExactSnapshot consistency option
• Applies the appropriate consistency to the request based on the provided options
• zedToken is ignored when using AnyFreshness / MustRefresh
• When zedToken is not provided and AtLeastAsFreshAs or AtExactSnapshot is specified, an exception is thrown

[tanczosm]

Good catch and better logic implementation. Any chance you'd be okay being added as a co-maintainer to this project? I am not using SpiceDb at the moment and the previous project I used it for did not get released. If you have some skin in the game I'd like to keep it going at least.