Add isValidPayload to AbstractAppliance

[slifty]

Issue #41

Description

This PR adds an isValidPayload method to AbstractAppliance which ensures that the payload is a valid type for the appliance.

It uses an assertion here (thus triggering an error) because submitting a payload of an invalid type for validation is likely a signal of a more serious problem.

Due Diligence Checklist

• [x] Tests
• [x] Documentation
• [x] Consolidated commits
• [x] Relevant labels assigned
• [x] Changelog(s) updated

Steps to Test

1. yarn test

Deploy Notes

None

Related Issues

Resolves #41

[slifty]

This really does warrant a review (just because it's an addition to an abstract class) but I'm going to press on in the name of speeeeeeeeeeed.