Add OIDC authentication support

[benoit-nexthop]

Summary

Adds OpenID Connect (OIDC) authentication support to GARM, allowing users to authenticate via external identity providers like Okta, Auth0, Google, etc.

Changes

• Add OIDC configuration to config.go with validation
• Add OIDC provider integration with state management and token exchange
• Add OIDC login/callback/status API endpoints
• Update NewUserParams with IsSSOUser flag for SSO users without passwords
• Consolidate CreateOIDCUser into CreateUser with IsSSOUser check
• Add OIDC login button to webapp login page
• Add OIDC tests and documentation

Configuration

To enable OIDC, add the following to your GARM config:

[oidc]
enabled = true
issuer = "https://your-idp.example.com"
client_id = "your-client-id"
client_secret = "your-client-secret"
redirect_url = "https://your-garm-instance/api/v1/auth/oidc/callback"

Testing

Tested with Okta in staging and production environments.

---

This PR is part of a stack of changes:

1. fix: remove invalid Unlock calls in scaleset worker #598
2. Add OIDC authentication support #599 ← you are here
3. Allow multiple users and open read-only APIs to non-admins #600
4. Add version display to navigation sidebar #601