Feature Request: Add --create argument to az pipelines variable-group variable update

- If the issue is to do with Azure CLI 2.0 in-particular, create an issue here at [Azure/azure-cli](https://github.com/Azure/azure-cli/issues)

### Extension name (the extension in question)
azure-devops
az pipelines variable-group variable update

### Description of issue (in as much detail as possible)
Add a flag --create to the command az pipelines variable-group variable update

az pipelines variable-group variable update --group-id
--name
[--detect {false, true}]
[--new-name]
[--org]
[--project]
[--prompt-value {false, true}]
[--secret {false, true}]
[--value]
[--create]

--create-c
If the variable does not exist, it can be created - otherwise just perform the update; similar to the functionality in helm.
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