Add php-amqp plugin

[jmjoy]

This pull request adds support for the php-amqp extension to the SkyWalking PHP Agent, enabling tracing of AMQP message publishing. It introduces a new plugin for AMQP, updates documentation and tests to reflect the new support, and ensures end-to-end coverage for AMQP scenarios.

AMQP Plugin Support:

• Added a new plugin_amqp.rs implementing tracing for the AMQPExchange->publish method, including span creation, peer extraction, and header injection.
• Registered the AmqpPlugin in the plugin system by updating mod.rs and the plugin initialization list. [1] [2]

Documentation Updates:

• Updated README.md to mark php-amqp as supported for message queuing producers.

Testing and Validation:

• Added a new end-to-end test case (amqp.php) to exercise AMQP operations and validate tracing.
• Integrated the new AMQP test into the test runner and added a helper function for the HTTP request. [1] [2]
• Updated expected test output to include AMQP spans in the trace context, verifying correct span creation and tagging. [1] [2]

Other:

• Removed the redundant cargo build step from the GitHub Actions workflow to streamline CI.