RabbitMQ: shared connections with context safety and config mismatch guard

[CassioGangrel]

Summary

• Add shared-connection-name support to reuse a single RabbitMQ connection across channels.
• Preserve incoming event-loop context even when a shared connection is created by outgoing first.
• Add a fingerprint check so channels using the same shared-connection-name must have identical connection settings.
• Add/adjust tests for shared connections and context.

Configuration

Example:

mp.messaging.incoming.data.shared-connection-name=shared-connection
mp.messaging.outgoing.sink.shared-connection-name=shared-connection

If two channels share the same name but have different connection settings, startup fails with:
Shared connection '<name>' has mismatched configuration...

Breaking changes / Documentation

• No breaking changes (feature is opt-in).
• Suggest documenting shared-connection-name and the config mismatch guard.
• Testing ./mvnw -pl smallrye-reactive-messaging-rabbitmq -DskipITs test

Simple flowchart:

flowchart TD
  IN[Incoming channel] -->|shared-connection-name| CONN[RabbitMQConnector]
  OUT[Outgoing channel] -->|shared-connection-name| CONN

  CONN -->|getClientHolder| POOL[sharedClients by name]
  POOL -->|compute fingerprint| FP[compare or create]

  FP -->|ok| CLIENT[(RabbitMQClient shared)]
  FP -->|mismatch| ERR[fail startup]

  CLIENT --> INCTX[Incoming event-loop context]
  CLIENT --> OUTCTX[Outgoing caller context]
  CLIENT --> MET[metrics/connection name using shared label]

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Fixes #3189