[RFE] Support for custom data (or document why it is not supported)

## Current situation
azure-init does not do anything with custom-data. See this [article](https://learn.microsoft.com/en-us/azure/virtual-machines/custom-data) for custom-data usage on Azure.

## Impact

Unlike user-data, custom-data is only available during provisioning phase because it's only available in the ovf-env.xml file. If the provisioning agent doesn't take action on custom-data, it will not be available again.

## Ideal future situation

Provide some options for how customers want to handle custom-data. A couple options
1) We save custom-data to a file and the customer chooses what to do with it.
2) We assume it's a script and execute custom-data like a script.
3) Detect that custom-data exists and print a WARNING message to recommend that customer use [user-data](https://learn.microsoft.com/en-us/azure/virtual-machines/user-data) instead.

## **Implementation options

__[ Optional: please provide one or more options for implementing the feature requested ]__

## Additional information

Differences between custom-data and user-data: custom-data is only available to privileged users as it's only available in the ovf-env.xml in a limited window during provisioning (the ovf-env.xml is only available through mounting of the provisioning ISO, which is a privileged action). user-data is available throughout the lifetime of the VM, and is accessible by anyone who has access to the VM. user-data can be updated throughout the lifetime of the VM while custom-data cannot be updated.