Improve handling of max_inbound_message_body_size (avoid hard-coded default; apply only when configured)

[Bharathi-Kanna]

This PR introduces support for max_inbound_message_body_size in the RabbitMQ input plugin with correct Behaviour:

• Adds the config option
• Applies the setting ONLY when the user explicitly configures it
• Removes the hard-coded fallback default from Add support for max_inbound_message_body_size parameter. Fixes #59 #62
• Allows the RabbitMQ Java client to use its built-in default (currently 64MB)
• Matches Behaviour of other RabbitMQ client integrations (Spring, NiFi, WSO2, Quarkus)
• Updates the documentation
• Adds tests for both configured and non-configured scenarios

This approach is future-proof, avoids unnecessary overrides, and ensures defaults stay aligned with the Java client.

Happy to refine based on feedback!

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
ecd676d

Please, read and sign the above mentioned agreement if you want to contribute to this project