Support Azure Pipeline Variable Groups in Push Extension

Currently we use the Push extension to seed configuration data when building new environments. We do not commit some of our configuration values to our repo because the data is considered sensitive.

To get around the current limitation in the push extension, we store the sensitive seed configuration file in the Pipelines "**Secure files**" library and then use a separate task to download the file before executing the Push task.

It would be incredibly helpful if we could just reference Pipeline **Variable Groups** directly.

Thanks!