Feature request: could use file for variables when `az pipelines variable-group create`

Or add batch mode/use file for `az pipelines variable-group variable create`
Sometimes there are too many variables(generated by script) which makes the command line longer than limit (https://support.microsoft.com/en-us/help/830473/command-prompt-cmd-exe-command-line-string-limitation)
Thanks~

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: ba39ac35-94ff-5cda-dede-4834d174ee4c
* Version Independent ID: 93f1ab7d-610b-0fc4-c247-c31dd7752f60
* Content: [az pipelines variable-group](https://docs.microsoft.com/en-us/cli/azure/ext/azure-devops/pipelines/variable-group?view=azure-cli-latest#ext-azure-devops-az-pipelines-variable-group-create)
* Content Source: [latest/docs-ref-autogen/ext/azure-devops/pipelines/variable-group.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/live/latest/docs-ref-autogen/ext/azure-devops/pipelines/variable-group.yml)
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**