Introduce macros for easier setup

[yakovmanshin]

• Instead of manually declaring the namespace, resolver, and individual flags, I’d like to have something less error-prone
• Additionally, code generation with macros allows to use async feature flags in an elegant way (something property wrappers aren’t capable of)

This is how I imagine the result:

#FeatureFlags(namespace: "MyFeatureFlags") {
    #Flag(key: "FFKey", name: "isEnabled", …)
    #AsyncFlag(key: "AsyncFFKey", name: "isEnabledAsync", …)
}