Add support for systemd-pkcs11 libcryptsetup plugin.

Add support for systemd-pkcs11 based LUKS2 device activation
via libcryptsetup plugin. This make the feature (pkcs11 sealed
LUKS2 keyslot passphrase) usable from both systemd utilities
and cryptsetup cli.

The feature is configured via -Dlibcryptsetup-plugins combo
with default value set to 'auto'. It get's enabled automatically
when cryptsetup 2.4.0 or later is installed in build system.