Add GetOrCreateCommandHandler for endpoint management

Introduced `GetOrCreateCommandHandler` to handle `GetOrCreateCommand` requests.
The handler checks if an `Endpoint` with the specified `Uri` exists in the repository
and returns it if found. If not, it creates a new `Endpoint`, saves it, and returns
the newly created object. Added necessary `using` directives for repository abstraction
and Entity Framework Core functionality.

src/ReC.Application/Endpoints/Commands/GetOrCreateCommand.cs.cs

@@ -1,4 +1,6 @@
-using MediatR;
+using DigitalData.Core.Abstraction.Application.Repository;
+using MediatR;
+using Microsoft.EntityFrameworkCore;
 using ReC.Domain.Entities;
 
 namespace ReC.Application.Endpoints.Commands;
@@ -7,3 +9,21 @@ public class GetOrCreateCommand : IRequest<Endpoint>
 {
     public string Uri { get; init; } = null!;
 }
+
+public class GetOrCreateCommandHandler(IRepository<Endpoint> repo) : IRequestHandler<GetOrCreateCommand, Endpoint>
+{
+    public async Task<Endpoint> Handle(GetOrCreateCommand request, CancellationToken cancel)
+    {
+        var endpoint = await repo.Where(e => e.Uri == request.Uri).FirstOrDefaultAsync(cancel);
+
+        if (endpoint is not null)
+            return endpoint;
+
+        endpoint = new Endpoint
+        {
+            Uri = request.Uri
+        };
+        await repo.CreateAsync(endpoint, cancel);
+        return endpoint;
+    }
+}