Fix: Remove Unnecessary &mut self Requirements from LightningNode Trait

[mubarak23]

closes #264
This PR refactors the LightningNode trait and its implementations to remove unnecessary &mut self references for methods that do not mutate internal state.

Motivation

Previously, several trait methods like get_network and list_channels required &mut self, even though they only retrieved information. This requirement leaked internal implementation details—specifically, that the underlying client field required a mutable reference to make gRPC or HTTP calls.

This design was misleading and unnecessarily restricted usage of the trait, e.g. preventing multiple read-only operations from being performed concurrently on the same node.

Changes

Wrapped client fields (e.g. NodeClient, EclairClient, etc.) inside tokio::sync::Mutex.

Updated all client usages to acquire a mutable lock via let mut client = self.client.lock().await;.

Updated trait method signatures to require &self instead of &mut self where applicable.

Ensured thread-safe, concurrent-friendly behavior without sacrificing correctness or clarity.

[carlaKC]

cACK, heading in the right direction 👍

[elnosh]

think should be good to squash

[carlaKC]

Small nit for when you do squash: take a look at commit structure in contributing guide - we use an odd format (historical reasons), would be nice to align with that while you're at it.

[carlaKC]

tACK, nice cleanup 🥦