Resolve "Add siac commands for /skynet/portals API endpoints"

MERGE REQUEST

MR Guidelines

Overview

Thought I should go ahead and do this since I already did #4005 (closed).

Non-interactive version where one command adds or removes one portal from the list. One possibility could be to support something like

siac skynet portals add portal1.com:1234 true portal2.com:4321 false

for adding multiple portals in one single command, but I don't see anyone wanting to generate such argument string if they use siac from their own application. Using the API directly would be more natural as taek mentioned.

Let me know if I should make any changes!

Example for Visual Changes

https://asciinema.org/a/CzjnYek1Axv9Ip6NDePzU5lha

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 #4004 (closed)