Refactor namespaces and introduce backdoor authentication

This commit refactors the namespace from `DigitalData.Auth.API.Dto` to `DigitalData.Auth.API.Models` in several files, improving the organization of data structures. A new `Backdoor` class is added to support backdoor authentication, along with a method in `DependencyInjection.cs` to register backdoor configurations. Additionally, `AuthApiParams` configuration is included in `Program.cs`, and a new JSON structure for backdoor users is introduced in `backdoors.json`. These changes enhance the codebase's structure and functionality.

src/DigitalData.Auth.API/Models/Backdoor.cs

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+namespace DigitalData.Auth.API.Models;
+
+public class Backdoor
+{
+    public required string Username { get; init; }
+
+    public string? Password { get; init; }
+
+    public string? PasswordHash { get; init; }
+}

src/DigitalData.Auth.API/Models/ConsumerLogin.cs

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+namespace DigitalData.Auth.API.Models
+{
+    public record ConsumerLogin(string Name, string Password);
+}

src/DigitalData.Auth.API/Models/DependencyInjection.cs

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+using Microsoft.Extensions.Options;
+
+namespace DigitalData.Auth.API.Models;
+
+public static class DependencyInjection
+{
+    public static IServiceCollection AddBackdoors(this IServiceCollection services, IConfiguration configuration)
+    {
+        var backdoors = configuration.Get<IEnumerable<Backdoor>>() ?? Enumerable.Empty<Backdoor>();
+        return services.AddSingleton(Options.Create(backdoors));
+    }
+}

src/DigitalData.Auth.API/Models/UserLogin.cs

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+namespace DigitalData.Auth.API.Models;
+
+public record UserLogin(string Password, int? UserId = null, string? Username = null);