cryptsetup: measure volume key and keyslot via the pcrextend Varlink service (#43109)

cryptsetup: measure volume key and keyslot via the pcrextend Varlink service (#43109)

Motivated by
https://github.com/systemd/systemd/pull/43041#discussion_r3595022610.

Switch systemd-cryptsetup's volume-key and keyslot measurements from
driving the TPM directly (tpm2-util) to the io.systemd.PCRExtend Varlink
service, aligning it with how the verity and imds measurements already
work.

Some notes on decisions taken:
- The volume key is sent over the wire. systemd-pcrextend will do the
hmac. The socket is root only. Otherwise we would need to do bank
negotiation via varlink and pollute the interface with it.
- `tpm2-measure-bank=` deprecated/dropped. Same reason as above.
- `tpm2-device=` now only affects unlocking. The device for measurements
is selected by pcrextend.
- Measuring requires the presence of `systemd-pcrextend.socket` in the
initrd, should be already given as systemd-veritysetup relies on it,
too.
- Logs are done on the pcrextend side.