Add option to make RpcClient fail early if there are no bindings

[janssk1]

I have noticed that the RpcClient utiltity has no way of using the 'mandatory' flag.
I would like my RPC call to fail 'fast' in case nobody is listening to the request. Wihout it, i have to wait for the RPC to timeout.

• RabbitMQ client version 5.5.0

[michaelklishin]

A return handler can be registered and either throw/etc or do nothing depending on user-provided values. @acogoluegnes WDYT?

[acogoluegnes]

This should be possible as-is:

• register a ReturnListener to the Channel instance used for the creation of the RpcClient.
• subclass RpcClient and override the publish method to publish with the mandatory flag set to true.

[michaelklishin]

Arguably the key part here is the ReturnListener implementation. If we could come up with a reasonably opinionated implementation that can be used by default the users would appreciate it.

[acogoluegnes]

It's straightforward to correlate the original blocking cell in the return listener, so we could unblock the blocking cell and throw an exception or do something else.