zdb: add ZFS_KEYFORMAT_RAW support for -K option

This change adds support for ZFS_KEYFORMAT_RAW to zdb_derive_key in
zdb.c. The implementation reads the raw key from the file specified
by the -K option which is consistent with how raw keys are handled in
the other parts of ZFS, along with a check to ensure that the keyfile
doesn't have too many bytes.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Patrick Xia <patrickx@google.com>
Closes #17783