Add parameter extension functionality for uprating factors

Summary

Currently, each country package (e.g., policyengine-us) needs to implement its own logic for extending uprating factors beyond the last year of official projections. This is a common pattern that would benefit from being standardized in policyengine-core.

Proposed Feature

Add a utility function to policyengine-core that can programmatically extend parameter values based on a growth rate calculated from the last years of available data. This would:

1. Calculate growth rate from the last N years of projections
2. Apply that growth rate to extend values through a specified end year
3. Handle different period types (monthly, yearly, etc.)

Example Usage

from policyengine_core.parameters import extend_parameter_values

# Extend a parameter through 2100 using growth from last 2 years
extend_parameter_values(
    parameter=parameters.gov.some.uprating,
    last_projected_year=2034,
    end_year=2100,
    period_month=10,  # For October values
    period_day=1,
)

Benefits

• Consistent approach across country packages
• Easier maintenance when projection years change
• Reduced code duplication
• More robust than hardcoding future values in YAML files

Current Implementation

A working implementation can be found in this PR: https://github.com/PolicyEngine/policyengine-us/pull/6384

The implementation includes:

• A generic extend_parameter_values function
• Support for different period types
• Calculation of growth rates from existing data

Related Discussion

This came up during the review of extending US uprating factors through 2100 for long-term policy simulations.

Start with the policyengine_core.parameters entry point and review the working implementation in policyengine-us PR 6384. Trace how parameter values and period types are represented, then define completion as a reusable extend_parameter_values utility that calculates growth from the last N projected years and extends values to the requested end year.