Add OIDC provider functionality with validation setup

This commit adds OpenID Connect (OIDC) provider functionality to tinyauth,
allowing it to act as an OIDC identity provider for other applications.

Features:
- OIDC discovery endpoint at /.well-known/openid-configuration
- Authorization endpoint for OAuth 2.0 authorization code flow
- Token endpoint for exchanging authorization codes for tokens
- ID token generation with JWT signing
- JWKS endpoint for public key distribution
- Support for PKCE (code challenge/verifier)
- Nonce validation for ID tokens
- Configurable OIDC clients with redirect URIs, scopes, and grant types

Validation:
- Docker Compose setup for local testing
- OIDC test client (oidc-whoami) with session management
- Nginx reverse proxy configuration
- DNS server (dnsmasq) for custom domain resolution
- Chrome launch script for easy testing

Configuration:
- OIDC configuration in config.yaml
- Example configuration in config.example.yaml
- Database migrations for OIDC client storage

go.mod

@@ -11,6 +11,7 @@ require (
 	github.com/gin-gonic/gin v1.11.0
 	github.com/glebarez/sqlite v1.11.0
 	github.com/go-ldap/ldap/v3 v3.4.12
+	github.com/golang-jwt/jwt/v5 v5.3.0
 	github.com/golang-migrate/migrate/v4 v4.19.1
 	github.com/google/go-querystring v1.1.0
 	github.com/google/uuid v1.6.0