Add dependency management for Rabbit's client

[philwebb]

When we do we should also consider #7499

[wilkinsona]

IIRC, we just get it transitively from Spring AMQP. Boot 2.0 is using Spring AMQP 2.0, so we've already upgraded. We should, perhaps, introduce dependency management for the client. It's in the Platform at the moment.

[philwebb]

Thanks @wilkinsona, that means #7499 can't happen in 1.5.