Add authentication

https://supabase.com/docs/reference/javascript/auth-signin

https://github.com/supabase-community/gotrue-csharp/blob/master/Gotrue/Api.cs

Start with the linked Supabase JavaScript auth-signin documentation and the GoTrue C# reference at Gotrue/Api.cs. Map the authentication surface needed by this Java client before choosing an implementation. Done should mean the requested authentication capability is implemented and its behavior matches the referenced Supabase API.

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