[FEATURE] RFC8628 support

[axxml]

Is your feature request related to a problem? Please describe.

It would be good to have support for RFC8628 for the OIDC workflow.

My use case is ssh login on headless devices via step-ca.

While step ssh login works with Tinyauth out of the box, step ssh login --console doesn't:

missing or invalid 'device_authorization_endpoint' in provider metadata
error generating OIDC token: exec "step oauth" failed

Describe the solution you'd like.

Implementation of OAuth 2.0 Device Authorization Grant allowing frictionless auth on headless devices.

Describe alternatives you've considered.

No response

Additional context

No response

Human Written Confirmation

• I confirm this request was written by me and not generated by an LLM or AI assistant.