Add GetOrCreateCommand for endpoint handling

Introduced the `GetOrCreateCommand` class in the `ReC.Application.Endpoints.Commands` namespace. This class implements the `IRequest<Endpoint>` interface from MediatR to handle requests for retrieving or creating an `Endpoint`.

Added `Uri` property to the command with an `init` accessor for immutability. Included necessary `using` directives for `MediatR` and `ReC.Domain.Entities`.

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

@@ -0,0 +1,9 @@
+using MediatR;
+using ReC.Domain.Entities;
+
+namespace ReC.Application.Endpoints.Commands;
+
+public class GetOrCreateCommand : IRequest<Endpoint>
+{
+    public string Uri { get; init; } = null!;
+}