[Feature Request] az pipelines variable-group update should support batch variable updating

**Is your feature request related to a problem? Please describe.**
az pipelines variable-group update should support batch variable updating. Current process to do update requires deletions and/or multiple additions.

The current code-base seems does not accept variable setup as input for `az pipelines variable-group update` even underlying SDK and API supports that.

**Describe the solution you'd like**
`az pipelines variable-group update` should get variables as optional input. Optionally passing data as JSON would be great.

**Additional context**
* https://learn.microsoft.com/en-us/rest/api/azure/devops/distributedtask/variablegroups/update?view=azure-devops-rest-6.0#request-body
* https://github.com/Azure/azure-devops-cli-extension/blob/master/azure-devops/azext_devops/devops_sdk/v6_0/task_agent/models.py#L3865-L3866