Preflight Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

Add support for POST method in UserInfo requests

Description

Currently, Dex's UserInfo implementation only supports the GET method when fetching user information from upstream identity providers. According to the OpenID Connect Core 1.0 specification (Section 5.3.1), the UserInfo endpoint MUST support both GET and POST methods.

Current Behavior

• Dex hardcodes the GET method for UserInfo requests
• Providers that require or prefer POST method cannot be properly integrated

Proposed Solution

Proposed Solution

Add a new configuration variable userInfoRequestMethod (or similar) to connector configurations that allows specifying the HTTP method for UserInfo requests.

Example Configuration

connectors:
- type: oidc
  id: example
  name: Example Provider
  config:
    issuer: https://example.com
    clientID: client-id
    clientSecret: client-secret
    redirectURI: https://dex.example.com/callback
    userInfoRequestMethod: POST  # New field: GET (default) or POST

### Alternatives Considered

_No response_

### Additional Information

_No response_