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/Configuration/ConfigurationHelper.cs

@@ -0,0 +1,32 @@
+using Microsoft.Extensions.Configuration;
+
+namespace OptixServe.Infrastructure.Configuration;
+
+public static class ConfigurationHelper
+{
+    public static IConfigurationBuilder CreateDefaultBuilder(string basePath)
+    {
+        var aspEnv = Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT");
+        var netEnv = Environment.GetEnvironmentVariable("DOTNET_ENVIRONMENT");
+        var env = aspEnv ?? netEnv ?? null;
+
+        var builder = new ConfigurationBuilder()
+            // .SetBasePath(Directory.GetCurrentDirectory())
+            .SetBasePath(basePath)
+            .AddJsonFile("appsettings.json", optional: true)
+            .AddJsonFile("config.json", optional: true);
+        
+        if (env != null)
+        {
+            builder.AddJsonFile($"appsettings.{env}.json", optional: true)
+                .AddJsonFile($"config.{env}.json", optional: true);
+        }
+
+        return builder;
+    }
+
+    public static IConfigurationBuilder CreateDefaultBuilder()
+    {
+        return CreateDefaultBuilder(Directory.GetCurrentDirectory());
+    }
+}