LighthouseClient: support heartbeats

[d4l3k]

Right now the bare bones coordination API doesn't support heartbeats (which is a notable oversight 😓). We should either move the heartbeating logic from manager.rs into a wrapper around the lighthouse client or we can do the simple thing for now and expose the heartbeat method on the pyo3 bindings.

https://github.com/pytorch/torchft/blob/main/src/lib.rs#L482-L484

[MattKotzbauer]

Am currently working on the simpler task of exposing the heartbeat method on pyo3 bindings - will open a PR shortly 🤙.