/skynet/stats endpoint

MERGE REQUEST

Overview

Implements a /skynet/stats endpoint which reports the number of files and total size uploaded to skynet.

Example for Visual changes (ie Screenshot)

Issues Closed

Closes #3876 (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