Introduce "height" field to Provider's `head` payload

[masih]

Calculating the height of advertisement chain is cheap and fast for a provider. That information can provide a significant cause in conflict resolution across indexers.

Indexers can also calculate this number while ingesting ads. However

• adding that on provider's side within the signed head payload reduces the need for trusting indexers.
• existing indexers who have not measured height do not need to re-ingest or re-traverse the ad chain to calculate this number.