Adds a new optional top-level oplog field h (document hash) to DurableReplOperation/oplog entries. This is only a schema change, no producer or consumer is wired yet. Downstream oplog parsers should tolerate the presence or absence of h.

Description of Linked Ticket

Context

Adds a new optional top-level field h (cpp name docHash) to DurableReplOperation in oplog_entry.idl. 
This ticket only adds the field to the oplog entry. Nothing should produce or read h yet.

This h field will eventually carry a hash of the document affected by an insert, update, or delete operation.

h must be optional. This is because older binaries, feature-off, FCV downgrade, and unsupported collections will produce oplog entries without it, and a required field would fail to parse those entries.

Acceptance criteria

• h added to DurableReplOperation as optional: true, as a int32 safeInt.
• Round-trip test: a DurableReplOperation without h parses without error.
• Round-trip test: a DurableReplOperation with h serializes and deserializes to the same value without error.
• No producer or consumer wired
• Marked downstream-impacting; tools team notified per the Downstream Impact process.