WIP: Support externally defined machine files.

[mrueg]

This is a first attempt to support externally defined machine files as well.

What the code does is:

• reads machineFiles key from config into machineFileConfigs
  for all items in this slice:
• figures out if this is a proper machineFile type (a map) or a pointer to an external file (string starts with "@")
• if it's a map, read it and add it to the actual machineFiles key in the struct
• if it's a string, find the file, and add them to a list of additionalMachineFiles that still need to be read.

for all items in the list of additionalMachineFiles

• check if it's a machineFile type (a map), read it and add it to the actual machineFiles key

Things that need to be checked:

• Tests are failing
• Figure out what the difference between LoadAndValidateFromFile and NewFromByte/NewFromFile is and why LoadAndValidateFromFile is not used in the tests/elsewhere.

[budimanjojo]

Thanks for the PR! ❤️

To answer the question in the checkbox, LoadAndValidateFromFile is calling NewFromByte, then substitute the all the ${} with the environment variables, then validate all the keys defined before returning the TalhelperConfig.

I'll have a look into your branch when I have time.

[budimanjojo]

Looking at the code, I think it's better to have custom UnmarshalYAML method for MachineFile like so: https://github.com/siderolabs/talos/blob/ccdb4c8b10450aa7fb6c32b0559bda73746a03ed/pkg/machinery/config/types/v1alpha1/v1alpha1_types.go#L840

I'm thinking about something like this in the ./pkg/config/config.go:

type NodeConfigs struct {
    MachineFiles   []MachineFileWrapper  `yaml:"machineFiles,omitempty"
    ....
}

type MachineFileWrapper struct  {
    MachineFile *v1alpha1.MachineFile
}

func (mfw *MachineFileWrapper) UnmarshalYAML(unmarshal func(any) error) error {
    // do stuffs here that returns mfw.MachineFile with type v1alpha1.MachineFile
}

So everything in the code can just be changed from MachineFiles to MachineFiles.MachineFileWrapper, but don't know how to do this 🤣