feat: Front channel embeds (#792)

<!--- Provide a general summary of your changes in the Title above -->

## Description

<!--- Describe your changes in detail -->

## Related Issue(s)

- #329 

## Verification

- [x] **Your** code builds clean without any errors or warnings
- [x] Manual testing done (required)
- [ ] Relevant automated test added (if you find this hard, leave it and
we'll help out)

## Documentation

- [ ] Documentation is updated (either in `docs`-directory, Altinnpedia
or a separate linked PR in
[altinn-studio-docs.](https://github.com/Altinn/altinn-studio-docs), if
applicable)

Co-authored-by: Knut Haug <knut.espen.haug@digdir.no>

src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommandValidator.cs

@@ -1,6 +1,7 @@
 using Digdir.Domain.Dialogporten.Application.Common.Extensions.Enumerables;
 using Digdir.Domain.Dialogporten.Application.Common.Extensions.FluentValidation;
 using Digdir.Domain.Dialogporten.Application.Features.V1.Common.Localizations;
+using Digdir.Domain.Dialogporten.Domain;
 using Digdir.Domain.Dialogporten.Domain.Common;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Actions;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Content;
@@ -171,6 +172,11 @@ public CreateDialogDialogElementUrlDtoValidator()
             .MaximumLength(Constants.DefaultMaxUriLength);
         RuleFor(x => x.MediaType)
             .MaximumLength(Constants.DefaultMaxStringLength);
+        RuleFor(x => x.MediaType)
+            .Must(MediaTypes.IsValid!)
+            .When(x => x.MediaType != null)
+            .WithMessage("Invalid media type, see docs for complete list <URL TDB>");
+
         RuleFor(x => x.ConsumerType)
             .IsInEnum();
     }

src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommandValidator.cs

@@ -1,6 +1,7 @@
 using Digdir.Domain.Dialogporten.Application.Common.Extensions.Enumerables;
 using Digdir.Domain.Dialogporten.Application.Common.Extensions.FluentValidation;
 using Digdir.Domain.Dialogporten.Application.Features.V1.Common.Localizations;
+using Digdir.Domain.Dialogporten.Domain;
 using Digdir.Domain.Dialogporten.Domain.Common;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Actions;
 using Digdir.Domain.Dialogporten.Domain.Dialogs.Entities.Content;
@@ -168,6 +169,10 @@ public UpdateDialogDialogElementUrlDtoValidator()
             .MaximumLength(Constants.DefaultMaxUriLength);
         RuleFor(x => x.MediaType)
             .MaximumLength(Constants.DefaultMaxStringLength);
+        RuleFor(x => x.MediaType)
+            .Must(MediaTypes.IsValid!)
+            .When(x => x.MediaType != null)
+            .WithMessage("Invalid media type, see docs for complete list <URL TDB>");
         RuleFor(x => x.ConsumerType)
             .IsInEnum();
     }

src/Digdir.Domain.Dialogporten.Domain/MediaTypes.cs

@@ -0,0 +1,45 @@
+using System.Net.Mime;
+using System.Reflection;
+
+namespace Digdir.Domain.Dialogporten.Domain;
+
+public static class MediaTypes
+{
+    // Custom MediaTypes for embedding content in frontend applications
+    private static readonly string[] CustomDialogportenMediaTypes =
+        [
+            "application/vnd.dialogporten.frontchannelembed+json;type=markdown"
+        ];
+
+    private static string[]? _validMediaTypes;
+
+    public static bool IsValid(string mediaType) => GetValidMediaTypes().Contains(mediaType);
+
+    private static string[] GetValidMediaTypes()
+    {
+        if (_validMediaTypes != null)
+        {
+            return _validMediaTypes;
+        }
+
+        var applicationMediaTypes = GetMediaTypes(typeof(MediaTypeNames.Application));
+        var imageMediaTypes = GetMediaTypes(typeof(MediaTypeNames.Image));
+        var textMediaTypes = GetMediaTypes(typeof(MediaTypeNames.Text));
+
+        _validMediaTypes = applicationMediaTypes
+            .Concat(imageMediaTypes)
+            .Concat(textMediaTypes)
+            .Concat(CustomDialogportenMediaTypes)
+            .ToArray();
+
+        return _validMediaTypes;
+    }
+
+    private static IEnumerable<string> GetMediaTypes(Type mediaTypeClass)
+    {
+        return mediaTypeClass
+            .GetFields(BindingFlags.Public | BindingFlags.Static)
+            .Where(field => field.FieldType == typeof(string))
+            .Select(field => (string)field.GetValue(null)!);
+    }
+}