Allow mapping cloud-init syntax to custom stages

This is a feature I have been wondering about for a while. Would be nice having some sort of mechanism to allow mapping cloud-init syntax to specific stages instead of being stuck to boot and initramfs.

One option would be adding an extra field like yip_stage or something similar, we could eventually write something like:

#cloud-config
write_files:
  - content: |
      created once connected to the network
run_cmds:
  - echo "I am online" 
yip_stage: network

The limitation of this is that it would not be possible to set multiple stages in a single file.

Other more sophisticated (and probably a bit convoluted) approach could be providing a map directly:

#cloud-config
write_files:
  - content: |
      created once connected inside the initramfs env
run_cmds:
  - echo "I am online" 
yip_stages:
  run_cmds: network
  write_files: initramfs

I was curious to know what you think about these approaches @mudler @Itxaka
Probably there are better options I did not think about

No files, tests, or entry points are named. Start by locating the existing boot and initramfs stage handling and cloud-init YAML processing, then compare the proposed yip_stage and yip_stages approaches; done requires an agreed mapping design with documented behavior and tests.