Improve plugin architecture for testing

[raehat]

Currently, when running the Barq plugin with Core Lightning, we need to use exclusive commands like barqpay to access payment functionality. However, it would be better to integrate our functionality into the default pay command of Core Lightning. This approach will:

• Simplify testing by reusing the existing Core Lightning infrastructure, which is crucial for ensuring that every test passes.
• Enhance the user experience.

[vincenzopalazzo]

To do this you can override the rpc_common with a plugin hook https://docs.corelightning.org/docs/hooks