Allow publishing plugins to require/provide auth

[benbrown]

Many publishing plugins (Azure, PVA, Github) will require authentication to a given service in order to carry out the actions.

It should be possible for the plugin to require a specific type of authentication, and for this authentication to be separate from the bulk "access restriction" authentication currently available through the auth plugin.

It should be possible, for example, to require auth to Github to publish to Github and have this handled as an in-app flow.

Describe the solution you'd like

• Allow a publishing plugin to return a 401 or 403 header and redirect to specific login page
• Introduce ability to expose auth endpoints but not require them for access (that is, do not apply them by default, just enable them to be used to unlock specific capabilities)
• Figure out a secure way for Composer to store login credentials like this

[benbrown]

This will be part of 1.3