Provide guidance on how to apply authentication conditionally

[timostamm]

In practice, it is common to have a handful of select RPCs that should be allowed to be called anonymously. We've had users asking for how to implement it with this module.

Since protoc-gen-connect-go generates a const for every procedure name, and authn.Request has a method to obtain a procedure name, it should be possible to implement an allowlist nicely.

It seems like we may want to make users aware of the existence of the generated const and the request method in the documentation, maybe with an example.