Add Alert system to top level modules

MERGE REQUEST

Overview

This MR lays the groundwork for an alert system that lets modules register alerts and users grab those alerts using a new API endpoint.

Changes to Work Flow

This MR adds a new interface modules.Alerter to Sia and adds it to all the modules together with an empty implementation. It also adds a new endpoint to the API /daemon/alerts.

Testing Changes

TestContractorIncompleteMaintenanceAlert was added to contractor_test.go.

Documentation Changes

API docs were updated and modules/README.md was created

Modules and Packages Impacted

Every module and submodule + node/api/client, siatest/renter, doc/api

Example for Visual changes (ie Screenshot)

No visual change

Issues Closed

No open issues for this MR

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