Deprecate AuthenticationProvider

AuthenticationManager and AuthenticationProvider have the same primary signature. In an effort to simplify the API, AuthenticationProvider should be deprecated.

Here is an initial list:

• Have existing AuthenticationProviders implement AuthorizationManager
• Add DelegatingAuthenticationManager to replace ProviderManager
• Introduce DSL support for specifying AuthenticationManager for each authentication mechanism
• Use AuthenticationManager by default for each authentication mechanism
• Deprecate authentication-provider XML support
• Deprecate DSL support for authenticationProvider()
• Consider creating AuthenticationProviderManagerAdapter to adapt AuthenticationProviders into AuthenticationManagers
• Consider @Bean support for multiple AuthenticationManagers