Add a way to update (`Set`) Key Vault using `.env` file

[bartvdbraak]

• Add boolean flag --set
• Add docs about what you need to do for it and which Access Policies
• Add code

[webbrain-one]

Hi! I’ve opened #182 to address this. It adds a --set flag that lets you update Key Vault secrets using a .env file, and includes docs covering the required setup and Access Policies. It should cover all the items in the issue checklist.