Exporting Ignition's `renderConfig` functionality

Feature Request

Exporting Ignition's renderConfig functionality so that external agents can resolve Ignition configs same way as Ignition engines does on nodes.

Desired Feature

Ignition uses renderConfig on machines to resolve remote sources for replace and append directives.

• right now renderConfig function is unexported
• Fetcher struct used by renderConfig can be vendored as it is part of internal package.

Exporting the renderConfig function would allow external agents to render an Ignition config that is complaint with Ignition's behaviour.

Specific Use Case

To manage the configuration for machines in Openshift clusters, there is plan to deploy an Ignition endpoint inside the cluster, that will serve Ignition config to machines. The config served to machines should be static and should have no remote sources.

Rendering the remote sources in the ignition config (that might have been provided by user) ensures that machines joining the cluster at a time in future continue to receive the same configuration even if the contents of the remote location have changed.

/cc: @crawford @yifan-gu

Start with internal/exec/engine.go at the linked renderConfig implementation and trace how its Fetcher dependency is used. Review the surrounding package API and existing tests before deciding what a supported exported entry point should expose. Done means external agents can invoke equivalent rendering behavior for remote replace and append sources without relying on an unexported function.