[cli] support adding multiple app names in cdk.json

support adding multiple app names in `cdk.json`
```
{
"apps": [
{
"name": "app1",
"command": "python dir1/app.py"
},
{
"name": "app2",
"command": "python dir2/app.py"
}
]
}
```
### Use Case

If you have multiple apps sharing the same `cdk.json` you either have to pass in the app via `cdk --app` or copy `cdk.json` around, it would be much nicer if we could support having a single `cdk.json` with multiple apps

### Proposed Solution

support adding multiple app names in `cdk.json`
```
{
"apps": [
{
"name": "app1",
"command": "python dir1/app.py"
},
{
"name": "app2",
"command": "python dir2/app.py"
}
]
}
```

### Other

* [ ] :wave: I may be able to implement this feature request
* [X] :warning: This feature might incur a breaking change

---

This is a :rocket: Feature Request

Start by reading the cdk.json configuration and the cdk --app entry point to understand how one app command is currently selected. Determine how the proposed named apps should be selected and how breaking-change behavior should work; done means a single cdk.json can define and use multiple named app commands, with CLI behavior and tests covering the configuration.

Written by the indexing model from the issue text.