Extend SkyfileMetadata To Include Len

MERGE REQUEST

MR Guidelines

Overview

This MR adds a length to the SkyfileMetadata (at root level) - avoiding the need to calculate it manually when we serve the file.

Example for Visual Changes

N/A

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 #4290 (closed) Closes #4295 (closed)