Amphora Plugins Auth Requirement

[jonwinton]

Description

Currently any Amphora plugin can only accept GET requests because of this line: https://github.com/clay/amphora/blob/master/lib/auth.js#L69

Reported by @jjpaulino

Steps to Reproduce

Steps to reproduce the behavior:

1. Make any plugin which accepts a request other than GET or OPTIONS
2. Execute a request to the plugin
3. See requirement for authentication

Expected Behavior

A plugin should be able to opt-out of authentication requirements

Additional Context

We should make the authentication requirement a default but allow plugins to opt-out. This will require moving the function which is enforcing this to be applied individually to core routes and then a test for plugin routes will need to be added.