Add ConfigMap support as FileSource for write_files configuration

[banschikovde]

Describe the solution you'd like:

Add support for ConfigMaps as a FileSource in write_files configuration for cloud-init. Similar to the existing SecretFileSource functionality, we need the ability to reference ConfigMap data when generating files during initialization. This will allow getting file content from either Secrets or ConfigMaps depending on the use case.

Why do you want this feature:

Currently file content can only be sourced from Secrets, which is limiting for non-sensitive configuration data
ConfigMaps are the standard Kubernetes way to handle non-sensitive configuration
This will provide more flexibility in managing configuration data
Users can better separate sensitive data (in Secrets) from regular configuration (in ConfigMaps)

Anything else you would like to add:

The implementation should maintain backwards compatibility with existing SecretFileSource
Only one source type (Secret or ConfigMap) should be allowed at a time for a given file
This enhancement follows the standard Kubernetes pattern of using ConfigMaps for configuration data
Documentation will need to be updated to reflect the new ConfigMap support