Support user-level OAuth plugin authentication

[olliestanley]

Will support plugins for which ai-plugin.json contains:

"auth": {
  "type": "oauth"
},

• [ ] Plugin has client ID and secret, securely store encrypted version of client secret, store client ID
• [ ] Mechanism for plugin author receiving verification token
• [ ] Chat UI support for authenticating with a plugin
• [ ] Redirect user to plugin auth URL
• [ ] When user is redirected back to OA, exchange received code for an access token
• [ ] Pass access token to frontend which will pass it back in future plugin-enabled inference requests

See OpenAI spec for OAuth plugin authentication.