add siac alerts command

MERGE REQUEST

Overview

Add new siac alerts command that prints out all alerts from the daemon.

Changes to Work Flow

N/A

Testing Changes

N/A

Documentation Changes

N/A

Modules and Packages Impacted

siac

Example for Visual changes (ie Screenshot)

Here's what it looks like:

tbenz9@spartan1:~/go/src/gitlab.com/NebulousLabs/Sia$ siac alerts
There are 1 alerts

Alert ID: 0
  Module:   contractor
  Severity: warning
  Message:  contractor is attempting to renew/form contracts, however the wallet is locked
  Cause:    wallet must be unlocked before it can be used

tbenz9@spartan1:~/go/src/gitlab.com/NebulousLabs/Sia$

Issues Closed

Checklist

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

• Pipeline Passing (this is for NDFs where tests were run in a loop)
• All new methods, or updating methods have clear docstrings
• Testing added or updated for new methods
• 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