tpm2-util: add generic helpers for sealing/unsealing data

tpm2-util: add generic helpers for sealing/unsealing data

These helpers tpm2_seal_data()/tpm2_unseal_data() are useful for
sealing/unsealing data without any further semantics around them. This
is different from the existing tpm2_seal()/tpm2_unseal() which seal with
a specific policy and serialize in a specific way, as we use it for disk
encryption.

These new helpers are more generic, they do not serialize in a specific
way or imply policy, they are just the core of the sealing/unsealing.

(We should look into porting tpm2_seal()/tpm2_unseal() onto these new
helpers, but this isn#t trivial, since the classic serialization we use
uses a merged marshalling of private/public key, which we'd have to
change in one way or another)