chore: Introduce dedicated runtime

[holzeis]

Annotating the function with #[tokio::main] killed the runtime after the function completes. Thus we had to block the run function indefinitely for the long living threads to keep going.

This change introduces a runtime managed outside of the function scope and thus allows the run function to return bringing the following advantages.

• We don't block a whole frb worker thread just to run the lightning node, sync tasks, background processor, etc.
• We are using a multi threaded runtime instead of the current thread
  • allowing to actually join the background processor without blocking all other tasks.
  • making better use of multiple cpu cores.
• We are not creating a new runtime on every async bridge call.