Add Netlify Auth0 integration

[martinjagodic]

Netlify Identity is deprecated, so we need a new recommended way of authenticating users.

This issue is for enabling Auth0, for a general discussion about deprecated Netlify Identity go here: #7419

Auth0 is the new recommended identity platform by Netlify: https://docs.netlify.com/integrations/auth0/

[samo4]

To my knowledge we have the following players in "single sign-on and identity" or "PKCE for OAuth 2.0 authorization" +business:

• Cognito
• Auth0
• Keycloak
• Azure B2C
• Okta
• Ping Identity
• ForgeRock

Ideally Decap should be able to use any of them for authentication and authorization. It appears there are bits and pieces floating around in our codebase, but everything ever so slightly on the "not usable without hacking" part of the spectrum.