feat(openwebui): model api key support

[cmdoret]

Goal

Allows users to define OpenAI API keys in openwebui to access models programmatically.

Summary:

• adds openwebui as a jwt provider in envoy.
• provide signing key for openwebui to mint and sign jwts

Advantage:

• lets users generate their own tokens from the UI.
• simple to use from user perspective.

Limitations

• openwebui is in the middle of every API request (bottleneck)
• tokens do not share the idp identities -> rate limits and billing not working
• more complex envoy policies, default catch-all rate limit, ...