tpm2-util: auto-detect supported PCR banks

tpm2-util: auto-detect supported PCR banks

Previously, we'd encode PCR policies strictly with the SHA256 PCR bank
set. However, as it appears not all hw implement those. Sad.

Let's add some minimal logic to auto-detect supported PCR banks: if
SHA256 is supported, use that. But if not, automatically fall back to
SHA1.

This then changes both the LUKS code, and the credentials code to
serialize the selected bank, along with the rest of the data in order to
make this robust.

This extends the LUK2 JSON metadata in a compatible way. The credentials
encryption format is modified in an incompatible way however, but given
that this is not part of any official release should be OK.

Fixes: #20134