tpm2-setup: add new early boot tool for initializing the SRK

This adds an explicit service for initializing the TPM2 SRK. This is
implicitly also done by systemd-cryptsetup, hence strictly speaking
redundant, but doing this early has the benefit that we can parallelize
this in a nicer way. This also write a copy of the SRK public key in PEM
format to /run/ + /var/lib/, thus pinning the disk image to the TPM.
Making the SRK public key is also useful for allowing easy offline
encryption for a specific TPM.

Sooner or later we should probably grow what this service does, the
above is just the first step. For example, the service should probably
offer the ability to reset the TPM (clear the owner hierarchy?) on a
factory reset, if such a policy is needed. And we might want to install
some default AK (?).

Fixes: #27986
Also see: #22637

src/shared/generator.c

@@ -741,7 +741,7 @@ int generator_write_cryptsetup_unit_section(
         fprintf(f,
                 "\n"
                 "DefaultDependencies=no\n"
-                "After=cryptsetup-pre.target systemd-udevd-kernel.socket\n"
+                "After=cryptsetup-pre.target systemd-udevd-kernel.socket systemd-tpm2-setup-early.service\n"
                 "Before=blockdev@dev-mapper-%%i.target\n"
                 "Wants=blockdev@dev-mapper-%%i.target\n"
                 "IgnoreOnIsolate=true\n");