Implement a command to list Deployment Groups and Targets for each one

AFAIK there's currently no way to retrieve the deployment groups within a Project (accessible from the *project page >> Pipelines >> Deployment Groups*) nor the list of targets for a given deployment group.

- Implement a command to list all deployment groups from a project, e.g.:

```
az pipelines dg list --org https://dev.azure.com/ -p
```

- Implement a command to show targets for a deployment group, e.g.:

```
az pipelines dg list-targets --org https://dev.azure.com/ -p --dg-name
```

Even better would be to be able to manage deployment groups from `az`, like create/delete a deployment group, add/remove a target to a deployment group, etc... but I guess this takes much more work.