Add AuthenticationConverter interface

[edeandrea]

Based on the discussion in #5328 add an AuthenticationConverter interface that is used within the AuthenticationWebFilter. This interface should look like

@FunctionalInterface
public interface AuthenticationConverter extends Function<ServerWebExchange, Mono<Authentication>> {

}

Then retrofit ServerFormLoginAuthenticationConverter, ServerHttpBasicAuthenticationConverter, and ServerOAuth2LoginAuthenticationTokenConverter to implement AuthenticationConverter.

Deprecate the existing

public void setAuthenticationConverter(Function<ServerWebExchange, Mono<Authentication>> authenticationConverter)

and add a new method

public void setAuthenticationConverter(AuthenticationConverter authenticationConverter)

[edeandrea]

I added #5689 for this