BRAKING refactor project structure.

Refactor: the project is now divided into a more clear structure, with **Infrastructure** and **Application** layers added.

Refactor: configurations are split into sections for different layers.

Fix: now EF Core related operations, such as migration, should be invoked in `OptixServe.Infrastructure`, with config file and data dir passed into `dotnet ef` command. See `OptixServe.Infrastructure/Utilites/DesignTimeDbContextFactory.cs` for details.

Fix: EF migrations are ignored in gitignore on purpose in early development.

OptixServe.Infrastructure/Utilites/DatabaseHelper.cs

@@ -0,0 +1,33 @@
+using Microsoft.EntityFrameworkCore;
+using OptixServe.Infrastructure.Configuration;
+
+namespace OptixServe.Infrastructure.Utilites;
+
+public static class DatabaseHelper
+{
+    public static string BuildConnectionString(DatabaseSettings dbSettings)
+    {
+        return dbSettings.Type switch
+        {
+            DatabaseType.Sqlite => $"Data Source={dbSettings.Host ?? "optixserve.db"}",
+            DatabaseType.MySQL => throw new NotSupportedException("MySQL connection is not yet implemented"),
+            _ => throw new NotSupportedException($"Database type {dbSettings.Type} is not supported")
+        };
+    }
+
+    public static void ConfigureDbContext(DbContextOptionsBuilder options, DatabaseSettings dbSettings)
+    {
+        if (dbSettings?.Type == DatabaseType.Sqlite)
+        {
+            var dbPath = dbSettings.Host ?? "optixserve.db";
+            var connectionString = $"Data Source={dbPath}";
+
+            options.UseSqlite(connectionString, b => b.MigrationsAssembly("OptixServe.Infrastructure"));
+
+        }
+        else
+        {
+            throw new NotImplementedException("Only SQLite database is currently supported");
+        }
+    }
+}