Migrate Redis stack prompts from survey to huh

[blsmth]

Summary

• Migrate Redis multi-AZ and instance class prompts from survey to huh
• Extract RedisMultiAZForm and RedisInstanceClassForm builders
• Replace BooleanOptionProxy with YesNoOptions + YesNoToBool

Closes #121

Changes

File	Purpose
stacks/redis.go	Replace ui.AskQuestions with huh forms; extract form builders
stacks/redis_test.go	teatest tests for multi-AZ and instance class selection

[blsmth]

@ipmb — Want to check if you're OK with us self-approving these survey-to-huh migration PRs to keep the project moving. Happy to wait for your review if you'd prefer, but wanted to flag it since these have been open a while.