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

It is possible to receive and send messaged directly to queues instead of exchanges, see [this guide](https://www.springcloud.io/post/2022-01/springboot-rabbitmq-rpc/#gsc.tab=0) 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.