Post process outgoing messages in JmsClient #35271
Description
So far, the JmsTemplate would use MessagePostProcessor for mutating JMS messages before they are being sent, but only if the method takes a post processor as an argument. The main use case so far is to mutate messages after they've been created by a MessageConverter from a payload.
As requested in #22999, developers would like to be able to mutate all messages before they're being sent, so as to add context information to the message. We can use the new JmsClient for that, and introduce support for MessagePostProcessor, this time for all outgoing messages (converted or not).
We should also consider updating the JmsClient static factoriesas a result, as this requires more configuration options.