homed: add key management toolchain (#36513)

if Lennart shall dogfood ParticleOS he needs acceptable tooling for
making his homed home dir accessible from his legacy fedora install, and
from local and remote particleos. Let's add explicit support for
scenarios like this:

1. add high level support for enrolling the account signing key from the
fedora install
2. add high level support for "adopting" a local but foreign .home file
on a system
3. add high level support for "registering" a remote user account on a
different system

(this lacks test cases and some docs, hence marked as wip)