Add RenewBaseCosts for RHP3 Renewal code

MERGE REQUEST

MR Guidelines

Overview

Example for Visual Changes

Both host and renter had their own code to compute the base costs for renewing contracts. Unfortunately as we changed the allowance multiple times, we also touched that code on the renter without updating the host. Since we are adding a new RHP3 RPC for renewing contracts I'm reimplementing some of that code in a shared location for both the renter and host to use with the new RPC. The old code remains untouched to avoid unnecessarily breaking it.

NOTE: This code will either be used once !4769 (merged) is merged or if this gets merged first the other MR will be updated.

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods or updated methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to workflow
• Issue added to Sia-UI repo for new supporting features
• Changelog File Created