Add SkykeyManager

MERGE REQUEST

Overview

Adds a new renter subsystem for managing skykeys.

A follow-up MR will implement API calls and siac commands.

Another follow-up MR will force XChaCha20 usage after !4125 (merged) is merged.

Example for Visual changes (ie Screenshot)

Issues Closed

2nd part of #3811 (closed)

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