[mod_amqp] Add XML handler via AMQP with RPC reply pattern

[alexey-khabulyak]

Description

Adds an XML handler for mod_amqp that allows FreeSWITCH to fetch XML configuration (directory, dialplan, etc.) from a remote backend over AMQP using the standard RPC reply_to/correlation_id pattern.

Architecture:

• A dedicated reader thread establishes its own AMQP connection, declares a broker-assigned exclusive auto-delete queue (amq.gen-...), and consumes replies
• A dedicated sender thread maintains a separate AMQP connection and publishes XML fetch requests to a configurable topic exchange, setting reply_to to the reader's queue name and a UUID as correlation_id
• Each xml_amqp_fetch call creates a per-request APR queue (capacity 1), registers it in a hash by correlation_id, then blocks until the reader thread dispatches the response or the fetch timeout expires
• The remote backend replies to the default exchange with routing_key = reply_to — no custom reply exchange needed

Removes mod_amqp_aux_connection_t entirely; the reader connection now reuses the existing mod_amqp_connection_t struct.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Code cleanup / refactor

Related Issues

Testing

• Tested manually — verified end-to-end XML fetch using a Python test responder (test_xml_handler.py) against a live RabbitMQ 4.x broker; user_data FreeSWITCH command returns correct values from the remote XML response

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the project's style guidelines
• I have added tests for my changes (if applicable)
• All existing tests pass

Additional Notes

The fetch timeout (fetch_timeout_ms, default 5000ms) and circuit breaker (circuit_breaker_ms, default 10000ms) are configurable via amqp.conf.xml. See updated conf/vanilla/autoload_configs/amqp.conf.xml for reference configuration.