Support for AuthenticationManagerBuilder injection into user code

Spring Boot provides a default AuthenticatiomManager for getting
started quickly with security and never exposing insecure
endpoints. To override that feature as users move to the next
stage in their project, they may have to do something slightly
different depending on whether it is a webapp or not.

In any app (web or not), providing a @Bean of type
AuthenticationManager always works, but you don't get the benefit of
the builder features.

In a webapp the user can also extend WebSecurityConfigurerAdapter
to provides a custom AuthenticationManager, and the preferred
way of doing that is via a void method that is autowired with an
AuthenticationManagerBuilder. The default AuthenticationManager is
built in a configurer with @Order(LOWEST_PRECEDENCE - 3) so
to override it the user's confugrer must have higher precedence
(lower @Order).

@EnableGlobalMethodSecurity can also be used in a non-webapp, and
Spring Boot will still provide a default AuthenticationManager.
To override it the user has to either extend
GlobalMethodSecurityConfiguration or provide a @Bean of type
AuthenticationManager (there's no other way to
capture the AuthenticationManagerBuilder that doesn't happen too late
in the beans lifecyle).

Fixes gh-244