ethexe: move NetworkMessage out of service module and consider other structure

[grishasobol]

File Location(s)

ethexe/cli/src/service.rs

Proposal

Move NetworkMessage to another module
Change to something like:

#[derive(Debug, Clone, Encode, Decode)]
pub enum NetworkMessage {
    Commitments {
           Publish { .. }
           RequestValidation { .. }
           Approve { .. }
    },
}