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 latest version.

Key features:

• Creates a new version with the specified start_date
• Updates only the provided fields - any field not specified will inherit the value from the latest version
• Validates that the new start_date is after the latest existing 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 creation succeeded event for the new version.