Auto-configure Rabbit connection factory with CredentialsProvider and CredentialsRefreshService when available

[garyrussell]

Consider adding support for OAUTH2 by auto-configuring CredentialsProvider and CredentialsRefreshService beans (if present and unique) into the RabbitConnectionFactoryBean.

2.4.x only; requires this PR to be merged: spring-projects/spring-amqp#1212

[mbhave]

@garyrussell Does that mean Spring Boot would autoconfigure the CredentialsProvider and CredentialsRefreshService beans or are you suggesting we set them on the RabbitConnectionFactoryBean only if a user provides them?

[garyrussell]

@mbhave I was just thinking that Boot should detect the presence of the @Beans and wire them in.

That said, there is only one (non-standard) implementation OAuth2ClientCredentialsGrantCredentialsProvider provided by RabbitMQ (and a DefaultCredentialsRefreshService) so I suppose we could configure them.

But I think it would be more flexible just to detect the beans - that way, the user can provide his/her own implementation.