Refactor Node interface #1896

Node interface is very cometbft centric. Apart from life-cycle methods it defines only one method - GetClient which returns cometbft RPC client implementation for given node type.

As rollkit won't expose cometbft client directly, we can reduce the interface to life-cycle methods which makes it kind of redundant (it will be almost equivalent to plain Service).

We need to rethink the Node abstraction, as we might want to expose some functionality of node (via RPC for go-execution implementations, or via go-interface for testing).