SecurityPkg/AuthVariableLib: Use EFI_CERT_DATA to parse certificate

SecurityPkg/AuthVariableLib: Use EFI_CERT_DATA to parse certificate

The function Pkcs7GetSigners return certificate stack as binary buffer.
Use EFI_CERT_DATA to parsing certificate stack more clearly, and access
certificate by the field of EFI_CERT_DATA structure.

Cc: Long Qin <qin.long@intel.com>
Cc: Zhang Chao <chao.b.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: chenc2 <chen.a.chen@intel.com>
Reviewed-by: Long Qin <qin.long@intel.com>
Reviewed-by: Zhang Chao <chao.b.zhang@intel.com>