Create interest plan version

Create a new version of an existing interest plan. This endpoint allows you to update specific fields while preserving others from the previous version.

Key features:

• Creates a new version with the specified start_date.
• Updates only the provided fields—any field not specified inherits the value from the previous version.
• Validates that the new start_date is after the start_date of the previous version.
• Maintains immutable fields like interest_type and tier_calculation.
• Preserves existing tiers when updating interest_by_tiers—only specified tiers are updated.

This endpoint generates an Interest plan created event.