Flutter: Custom Provider Admin

• Methods under the supabase.auth.admin.customProviders namespace manage custom OIDC/OAuth providers programmatically. Requires a secret key.
• These are admin methods and should be called on a trusted server. Never expose your secret key in the Flutter app.
• Custom providers are referenced with a custom: prefix when signing in (for example custom:mycompany), and are distinct from the OAuth 2.1 server clients managed through supabase.auth.admin.oauth.