[CLI]: `dab update` option to enable/disable multiple-mutations

To simplify usage, is it possible to have:

`"nested-mutations" : true`
to imply any supported nested mutations are enabled. Could be a separate PR.

_Originally posted by @Aniruddh25 in https://github.com/Azure/data-api-builder/pull/1983#discussion_r1508613872_

Introduce a new option to enable/disable all nested mutation operations through CLI.

The resulting JSON in the config file would be of the form

```json
"runtime":{
...
"graphql": {
...
"multiple-mutations": true/false
}
}
```

_Note_: The exact name of the field - `multiple-mutations` might change as the new name for the feature is under discussion. Will update this issue later with the final name of the field.