Make provenance in seasonal snapshots consistent with forecast and real-time

[aidan-short-gevernova]

• Updated seasonal snapshots schema to include a provenance array to match forecast and real-time. Each provenance item will contain a resource id and a proposals-considered array.
• Updated the max size of the proposals-considered array from 12 to 48 to support the possibility of 12 periods with 4 proposals each. This follows the assumption of 4 proposals per period made in forecast proposals considered.
• In addition to source and resource-id being required for each proposal-considered item, I also made period-start, period-end, cotinuous-operating-limit, and emergency-operating-limits required to match existing schemas.
• Updated the seasonal snapshot examples to match the new schema.
• Also adds custom seasonal snapshot header types rather than directly using common-header and base-header to match forecast and real time.

The spec looks like the following after the changes:

Closes #252