SecurityPkg/Pkcs7Verify: Complete the Pkcs7VerifyDxe protocol

SecurityPkg/Pkcs7Verify: Complete the Pkcs7VerifyDxe protocol

VerifySignature can be implemented using a mirror of the
AuthenticodeVerify function that's already in use in the
ImageVerificationDXE environment, so this patch simply wires up
VerifySignature using that code.
<NOTE: Only Authenticode-signature verification was supported by
       this VerifySignature() implementation now.)

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: Long Qin <qin.long@intel.com>