[Feature Request] az pipeline variable-group importing

### Issue

When exporting variable-groups via Devops cli using the following command -
az pipelines variable-group show --group-id # -o yaml (Gets individual group variable) or if using this - az pipelines variable-group list -o yaml (Lists all the variable groups in one)

You end up with a long list of yaml code that you are unable to import, the same is if creating a new Variable Groups, this is more of an issue when moving DevOps accounts so doing an Export to then do a import. There appears to be no way to import this information.

### Feature

When creating large ranges of names and values to a new or update Pipeline variable-group, rather than putting these in each time, could a yaml or json file be used to import the names and values, as doing this with out this you manually enter the cli each time.

With multiple variables being required would mean entering in them multiple times, when there is only a few lines to import this isn't so much of an issue but when a yaml/json file could be over 200 lines doing this manually each time could become laborious and time consuming, so could something like the following be added?

i.e. –
az pipelines variable-group create -f c:\Azure\testyaml.yaml