Add a label parameter when creating pull requests

**Describe the solution you'd like**
A parameter to add labels to a pull request when creating it with the `az repos pr create` command.

Example:
```bash
az repos pr create \
--title "A cool pr" \
--description "With a cool description" \
--label "documentation"
```

**Additional context**
The python module azure-devops is already able to create pull request with labels: [Reference 1](https://github.com/microsoft/azure-devops-python-api/blob/7f2d7a489ad5d305512ff6f5337ca14605904531/azure-devops/azure/devops/v7_1/git/git_client_base.py#L2140), [Reference 2](https://github.com/microsoft/azure-devops-python-api/blob/7f2d7a489ad5d305512ff6f5337ca14605904531/azure-devops/azure/devops/v7_1/git/models.py#L1472)