base implementation for download-by-root

MERGE REQUEST

Overview

This MR implements code to download a sector root from the network without knowing which host has that sector root. The code is clean and ready to merge, though from a pure cost and performance perspective, there is room for improvement. These performance and cost gains will be pursued after the MDM, EA, and multiplexers are available.

There is testing on the download-by-root branch where there is more code written. I broke this out so that the download-by-root branch didn't need to be merged all as one giant ball of code.

Checklist

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

• All new methods, or updating 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 work flow
• Issue added to Sia-UI repo for new supporting features
• Changelog updated