Support service-level plugin authentication

[olliestanley]

This will support plugins for which ai-plugin.json has auth.type of "service_http". Plugin code will have a service access token.

• [ ] Securely store an encrypted service access token
• [ ] Send service access token in the Authorization header for plugin requests
• [ ] Create mechanism for plugin creators to get an OA verification token

See OpenAI spec for service level auth.