fail: add a runtime codegen check

[lucab]

This adds an helper function which allows consumers to check (at runtime) whether the library was compiled with the (buildtime) failpoints feature, which enables code generation for failpoints.

[lucab]

/cc @BusyJay @overvenus @brson for review.

This enables all consumers to introspect library buildtime configuration in a uniform way (i.e. regardless of feature renaming in case of intermediate consumers). It is mostly sugar for consumers, so feel free to reject if you don't like it.

[brson]

Thanks @lucab