Add Network boot option: boot to DTS

Nobody has claimed this yet.

Stakeholder so end users can boot to Dasharo Tools Suite ([DTS](https://docs.dasharo.com/osf-trivia-list/dts/))

Network booting into DTS is a low hanging fruit, both for PoC but also because DTS is feature rich firmware swiss army knife.

Depends most probably on #1936 so HTTPS support enforced properly

- [ ] Have proper SSL/TLS support to download HTTPS content
- [ ] Do PoC to download initramfs+kernel + boot into it
- [ ] Refactor network-init-recovery so that functions are moved out to /etc/functions, call those functions from network-init-recovery
- [ ] Add Network boot menu option
- [ ] Add DTS boot option calling /etc/functions refactored functions
- [ ] think about how to bypass lock_chip properly, proper UX for that (warning good enough to deactive env variable dynamically?)
- [ ] Push Dasharo to detach sign their kernel+initrd so Heads can verify signatures, not rely solely on HTTPS
- [ ] Do PR

Start with issue #1936 for the HTTPS prerequisite, then inspect network-init-recovery and the proposed /etc/functions refactor. Trace the existing network boot menu and recovery flow before scoping the DTS initramfs and kernel path. Done means the checklist is implemented, including the menu option, DTS boot handling, signature and lock_chip UX decisions, and a PR.

Written by the indexing model from the issue text.