Design Network and Sync implementations

[timwu20]

Issue Summary

From design doc:

The actual implementation used to fulfill Network and Syncing interfaces using existing Gossamer peerset, synching, and network modules will be outlined in another design document specifically focused on integrating the GRANDPA client into the rest of the Gossamer codebase.

Acceptance Criteria

• Design doc to outline scope of work to implement both interfaces
• Creation of issues
• NOTE: Implementation is blocked on Implement GossipEngine #4522, which will introduce these interfaces into the codebase.