Make CodeVerifierAuthenticator public

[Vojtech-Sassmann]

Expected Behavior
The CodeVerifierAuthenticator is public and can be used in custom AuthenticationProviders.

Current Behavior
The CodeVerifierAuthenticator is package-private and cannot be used outside this package.

Context
We are implementing a custom AuthenticationProvider which uses client X509 certificate. We also would like to use PKCE inside this AuthenticationProvider. We cannot simply reuse the CodeVerifierAuthenticator because its visibility is restricted. We must either write custom PKCE validation and duplicate the logic or create the custom AuthenticationProvider inside the same package as the CodeVerifierAuthenticator.

Related PR: #1348