As part of the ASYNC step-down initiative, MongoDB needs a way to inform WiredTiger of the exact cutover time for a planned step-down.

Instead of creating a new API, we are extending the existing set_timestamp API to accept a new timestamp key (step_down_ts). This will act as the initial integration point for the Server team, unblocking them to begin their step-down coordination logic while WT implements the underlying ingest/stable routing logic.

Acceptance Criteria / Tasks:

• Modify set_timestamp to parse and accept the step_down_ts=<timestamp>.
• Store the provided step_down_ts internally (following the existing pattern used for the oldest and stable timestamps).
• Introduce an internal flag to track that the node has entered the "prepare-to-step-down" transition phase.
• Update the step-down execution path to clear this transition flag once the step-down is complete.