Introduces a new peer-to-peer messaging plugin to the RMQTT ecosystem.

**Changes Made:**
* Added new plugin dependency `rmqtt-p2p-messaging` in workspace Cargo.toml
* Updated plugin version to 0.16.0 in package metadata
* Included the plugin in rmqtt-bin executable dependencies
* Added plugin to plugins Cargo.toml features (full and p2p-messaging)
* Registered plugin in plugins library exports
* Added plugin to default startup configuration in rmqtt.toml
* Included plugin registration example in plugins example code

**Files Modified:**
* Cargo.toml - Added new plugin dependency
* rmqtt-bin/Cargo.toml - Added plugin to binary dependencies
* rmqtt-plugins/Cargo.toml - Added plugin feature flags
* rmqtt-plugins/src/lib.rs - Added plugin module export
* rmqtt.toml - Added plugin to default startup list
* rmqtt/examples/plugins/src/main.rs - Added plugin registration example

This new plugin enables peer-to-peer messaging capabilities within the RMQTT broker ecosystem.