tpm2-util: add common parser for the LUKS2 TPM2 JSON structure

tpm2-util: add common parser for the LUKS2 TPM2 JSON structure

This splits out the JSON parser used by the systemd-cryptsetup code.

This is preparation for later work to reuse it in the tpm2 cryptsetup
token module, which currently uses a separate but very similar parser
for the same data.

No change in behaviour.