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

internal/model/oidc_client_model.go

@@ -0,0 +1,18 @@
+package model
+
+type OIDCClient struct {
+	ClientID     string `gorm:"column:client_id;primaryKey"`
+	ClientSecret string `gorm:"column:client_secret"`
+	ClientName   string `gorm:"column:client_name"`
+	RedirectURIs string `gorm:"column:redirect_uris"` // JSON array
+	GrantTypes   string `gorm:"column:grant_types"`   // JSON array
+	ResponseTypes string `gorm:"column:response_types"` // JSON array
+	Scopes       string `gorm:"column:scopes"`         // JSON array
+	CreatedAt    int64  `gorm:"column:created_at"`
+	UpdatedAt    int64  `gorm:"column:updated_at"`
+}
+
+func (OIDCClient) TableName() string {
+	return "oidc_clients"
+}
+