Feature Request: Introduce Custom Pathfinding in SimNode

**Describe desired feature**

Right now `sim-node` simply uses LDK's default pathfinding to find routes for payments that are dispatched in the mocked network.

To vastly expand the use of sim-ln as a library for pathfinding-related use cases, we should provide a way for calling code to provide custom routes instead.

This could be implemented a few ways:
1) Add an interceptor similar to #255 that sends proposed route and gets back modified one
2) Provide custom pathfinding in on creation of mocked nodes

Part of this issue will be to spend some time figuring out the best way to implement this.

**Use case for feature**

This opens up a lot of doors for A/B testing different pathfinding strategies.

**Would you like to contribute code for this feature?**

I'll review design proposal + code, unlikely to have time to do the code myself!