Add background service and update project configuration

Added a `Worker` class as a hosted background service to log
periodic messages. Updated `DigitalData.EmailProfiler.API.csproj`
to include `UserSecretsId` for secure development storage and
added `Microsoft.Extensions.Hosting` package. Replaced the
`Controllers` folder reference with `Properties`. Updated
`Program.cs` to register the `Worker` service, enable API
exploration, and retain Swagger configuration.

src/DigitalData.EmailProfiler.API/Program.cs

@@ -1,9 +1,10 @@
-var builder = WebApplication.CreateBuilder(args);
+using DigitalData.EmailProfiler.API;
 
-// Add services to the container.
+var builder = WebApplication.CreateBuilder(args);
+builder.Services.AddHostedService<Worker>();
 
 builder.Services.AddControllers();
-// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
+
 builder.Services.AddEndpointsApiExplorer();
 builder.Services.AddSwaggerGen();