Add option to make routing-key part of RabbitMQ transaction/span names

[JonasKunz]

It is possible to receive and send messaged directly to queues instead of exchanges, see this guide for example.

We currently only use the exchange for naming transactions and client spans, which is sub optimal in this case. We should provide an option to use the RabbitMQ routingKey instead.

In my opinion we should offer an enum-like configuration option (e.g. rabbitmq_naming_mode) to do this. This way we won't end up with multiple boolean options in case we see the need to further customize the naming scheme.

[JonasKunz]

Support for the non-spring client has been implemented via #3636, the option is not yet supported for spring.