[feat] Support reading parameters from different contexts

We talked about this offline, but I wanted to open a tracking issue since I'll likely begin on this support soon. For `azd up` and `azd provision` we should support `-p` / `--parameters` to pass in parameters. On typical linux shells this is otherwise easy:

```bash
FOO="a" BAR="true" azd up
```

However, on Windows - using either cmd or powershell/pwsh - you need to explicitly set env vars then execute the command. It also all feels more like a workaround, and assumes that all the parameters you want to pass are in `main.parameters.json`, which may not be the case.

IMO, anything passed to `--parameters` should take precedence over env vars (either OS env vars, or from _.azure/{environment}/.env_ since they are more specific i.e., they were specified explicitly on the command line.