Make MDM aware of contract length

MERGE REQUEST

Overview

• Add contract length to MDM
• Add StoreLengthCost which is price / byte / block
• Update MDMAppendCost to accept a storage duration

Currently a couple TODOs left in, will fix them once I get some feedback.

Example for Visual Changes

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

Issues Closed

Closes #4019 (closed)