Introduce a new radicle-metrics crate that provides metrics collection for COBs. This introduces a CobHistoryWalk structure allowing to traverse history of cob actions similar to rad cob log command but allowing for further aggregation and metrics derivation.

Crate contains rad-metrics CLI showing main use cases of proposed approach. The tool supports both one-time scans and continuous polling modes, making it suitable for both ad-hoc analysis and continuous monitoring.

Related-to: 68460fc9b3e7decbc7e61020519656248a9ef4ce Related-to: https://radicle.zulipchat.com/#narrow/channel/369277-heartwood/topic/radicle-metrics