Support Custom Routing Strategies in Multiple Languages

`barq` is structured into different crates, each serving specific purposes, which allows for future feature expansions.

One proposed feature is enabling users to create custom routing strategies in their preferred programming languages.

This should be straightforward, as the routing algorithm only needs to accept `RouteInput` and return the route hops wrapped in `RouteOutput` to `barq-plugin` to process the payment.

Start by mapping the repository's crates and reading the barq-plugin interface around RouteInput and RouteOutput. Determine how a custom routing strategy could cross the language boundary and what integration points are required. Done should allow a user to provide a strategy in another language that returns route hops for payment processing.

Written by the indexing model from the issue text.