feat(zenoh): Support multiple plugin instances

[zhexuany]

Feature Request

Allow multiple Zenoh plugin instances with different configurations.

Motivation

The global session pattern prevents running multiple Zenoh configurations in a single process. This limits use cases like:

• Connecting to multiple Zenoh routers
• Different QoS settings per subscription
• Isolated plugin instances for testing

Proposed Solution

1. Remove global session pattern
2. Make ZenohPlugin fully instantiable
3. Pass session through plugin interface

Breaking Changes

Yes - this would require updating the plugin ABI

Files to Modify

• middlewares/zenoh/src/zenoh_plugin/include/zenoh_plugin.hpp
• middlewares/zenoh/src/zenoh_plugin/src/zenoh_plugin.cpp

Priority

Low - Single instance is sufficient for current use cases