Design initramfs add-on entrypoint for bootc

UKI's give us the ability to do UKI add-ons but for ostree back-end and non sealed environments we should support something similar for initramfs config and content. The current rpm-ostree/ostree solutions pre-date bootc and do not work with bootc update.

ostree request on: https://github.com/ostreedev/ostree/issues/3634

Start by reading the linked ostree request (#3634) and the bootc update behavior described in the issue. Trace how bootc currently handles initramfs configuration and content, if documented in the project, and define what an add-on entrypoint must provide for ostree-backed, non-sealed environments to work with bootc update.

Written by the indexing model from the issue text.