Add .devcontainer and devcontainer.json for Codespace

[swigerb]

Create a .devcontainer directory in the root of the repo.
Add a devcontainer.json file that specifies a base image with azd, or installs azd during setup.
devcontainer.json:

{
  "name": "Azure Samples: Adaptive RAG Workbench Codespace",
  "image": "mcr.microsoft.com/devcontainers/universal:2.2",
  "features": {
    "ghcr.io/azure/azure-dev/devcontainers/azd:latest": {}
  },
  "postCreateCommand": "azd version",
  "customizations": {
    "vscode": {
      "extensions": [
        "ms-azuretools.azure-dev",
        "ms-azuretools.vscode-azurefunctions",
        "ms-azuretools.vscode-docker"
      ]
    }
  }
}