Cookie entrypoint webhook for custom cookies

[byewokko]

• The POST /cookie/entry request should trigger a webhook and obtain a response with additional cookies to set.
• The webhook URL should be configured in Client metadata.

This will require new client features

• cookie_entry_uri - considered as one of the Client's allowed Redirect URIs.
• cookie_webhook_uri - called by the cookie entrypoint after successful auth. Expected to accept a PUT JSON request and to respond with JSON containing cookie names and values.

Introspection, in case of 401, can automatically build the authorize?client_id=...&state=...&...&redirect_uri=<cookie_entry_uri> and send it back in the Location header. Nginx extracts it and does the redirection.

[byewokko]

resolved in #188