credentials: add policy that can allow key=null creds from the ESP (#42555)

This PR only sets the default to "relaxed" - I can change the default
to "tofu" if desired. But for that we will also need to update the NEWS
file to ensure everyone is aware of this new default.

---

This PR adds a new `systemd.credentials-boot=` kernel
commandline that allows to control if credentials with
a `null` key are accepted.

The possible options are:
* strict: always insist on tpm encryption
* tofu: allow null encryption in firstboot mode and when no tpm is
available
* relaxed: allow null encryption when sb is off, or no tpm is available
* off: allow null encryption always

The default is `relaxed` which is exactly the behavior we had before.

This replaces the initial idea of using plaintext credentials
at firstboot (thanks to Lennart for this nicer and simpler design).

---

With that we can drop `- firstboot: optionally accept credentials at
firstboot without authentication` from TODO.md

diff --cc src/core/main.c
Simple merge

diff --cc src/shared/condition.c
Simple merge

diff --cc src/shared/creds-util.c
Simple merge