Feature Request: Investigate Use of Existing Impl Pathfinding

**Describe desired feature**

When running with a fully simulated network, it would be amazing of SimLN could use the same pathfinding as LND/CLN/Eclair. LND would be the highest priority, because it's the majority of the network.

### Possible Approaches

#### Extern C

* Create C libraries that implement each pathfinding algorithm
* CLN: easy, compile library in C
* LND: use `c-shared` to build C shared library
* Eclair: not possible, so far as I know
* Use [extern](https://doc.rust-lang.org/std/keyword.extern.html) to call C code for each pathfinding

This would require some (likely painful) investigation to figure out if the `c-shared` approach would work for LND. There are all sorts of caveats about C-compatible types that I haven't looked into.

#### IPC

* Create "stub" servers that call directly to the implementations routing code:
* Listen on socket, accepting parameters expressed in json
* Run pathfinding
* Return result
* SimLN then spins up each of these small servers using [Command](https://doc.rust-lang.org/std/process/struct.Command.html)
* Implement the `Pathfinder` trait in #273 which is responsible for shooting the request into the socket

Notes:
* I don't have strong opinions on how to handle IPC (never done this myself)
* We _could_ also define a common RPC for these mini-servers, but that's a lot of boilerplate
* Main priority is that this should be ✨ seamless ✨ for the end user, SimLN should handle all the ugly stuff.

**Use case for feature**

A common use case for SimLN's random payment generation is to create "background noise" payments in a network, and then take and measure some action within that busy network.

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

I don't have capacity to take this on right now, but would be happy to participate in design discussions and review.