Add .devcontainer and devcontainer.json for Codespace

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:
```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"
]
}
}
}