Add node rewards endpoint to Node Metrics canister

[pietrodimarco-dfinity]

Added

• node_rewards endpoint to fetch rewards with/without consecutive days penalty
• Tested node rewards computation

Changed

• Compute daily proposed/failed blocks on write before storing metrics
• Modified key stable BTreeMap to be (ts, node_id)
• Normalized node metrics struct stored as valued in stable BTreeMap.
  Normalization helps with the aggregations served by the API, now read-heavy on node rewards endpoint.

[DFINITYManu]

wen svelte? :-D