Added project references to establish dependencies between
the API, Application, Domain, and Infrastructure projects.
Updated `DigitalData.EmailProfiler.Tests.csproj` to include
references to all layers for testing purposes. Fixed a BOM
encoding issue in the test project file. Added xUnit usage
directive to ensure proper test framework integration.
A new test project, `DigitalData.EmailProfiler.Tests`, has been added to the solution. The project is configured as a .NET 8.0 test project with xUnit as the testing framework. It includes necessary NuGet dependencies such as `coverlet.collector`, `Microsoft.NET.Test.Sdk`, and `xunit.runner.visualstudio`.
The solution file has been updated to include the new project, along with its build configurations (`Debug|Any CPU` and `Release|Any CPU`). A new solution folder, `tests`, has been added, and the test project is nested under it.
