Option to configure callback URL host for a provider

[nogginbox]

• Configurable callback URL host: Added CallbackUrlHost option to ProviderConfig so the host/port used in OAuth callback URLs can be explicitly set. Useful when running behind a reverse proxy (e.g. YARP, NGINX) where auto-detection doesn't work.
• ILogger per provider: Each provider now receives a typed ILogger<T> (e.g. ILogger<GitHubProvider>) via ILoggerFactory in the ProviderFactory, so providers can log with proper category names.
• Nullability cleanup: Tightened nullability annotations on AuthConfigSection, ProviderConfig, and JsonSerializerExtensions.

Test plan

• All 21 existing tests pass
• Verify callback URL host override works end-to-end behind a reverse proxy
• Confirm logging output appears with expected category names