Skyfile Backup and Restore

MERGE REQUEST

MR Guidelines

Overview

This MR adds backing up and restoring a skyfile. To enable the restoration the SkyfileLayout is now returned with the download.

The backup method resides in the client package since it simply downloads the skyfile data and saves it to disk.

The restore method adds a new API route and methods to the renter package.

Testing was added for cases mirroring those of the TestSkynetDownloads.

TODO siac commands will be handled in a follow up since the format of this MR has been changing.

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 #4467 (closed)