Verify the timestamp signature

[woodruffw]

In addition to the signature and certificate chain, we should also verify the timestamp signature embedded in each Authenticode SignedData.

Documentation for these is a little inconsistent, but they seem to be mostly standard PKCS#9 countersignatures:

• https://docs.microsoft.com/en-us/windows/win32/seccrypto/time-stamping-authenticode-signatures?redirectedfrom=MSDN

osslsigncode's codebase will probably be helpful here.