Add .NET Core SDK devcontainer

[Dedac]

This pull request adds a devcontainer configuration for .NET Core SDK 3.1. It includes a .devcontainer directory with a devcontainer.json file that specifies the use of the mcr.microsoft.com/dotnet/core/sdk:3.1 Docker image. The configuration also includes settings for mounting the workspace and installs necessary extensions for .NET Core development. For more details, please refer to the Copilot Workspace session.