Add command to show details about a signed object

[JohnAZoidberg]

It's useful to know whether an executable is already signed and by whom. Either to make sure that you have properly signed it or to check if somebody else has signed it.

Example:

> java -jar jsign/target/jsign-7.2-SNAPSHOT.jar show foo.exe
  jsign show foo.exe
  Signature 0
  Digest Algorithm:   SHA256
  Digest Value:       c481bb3892d066ffacba0650adaa4c252580b776b1dd6026cf4a8bea6c813939
  Is Timestamped?     false
  Certificate
    Subject:          CN=net.jsign.signing-cert
    Issuer:           CN=net.jsign.issuing-cert
    Not Before:       Fri Dec 03 14:34:46 CST 2021
    Not After:        Wed May 24 14:34:46 CST 2119
    Expired:          false
    Serial:           148957645726085760686199624248870688956

[JohnAZoidberg]

I'm adding support for this because sometimes I have some old binaries and need to check whether they were signed and with which certificates.

Additionally it would be useful to be able to verify if the signature is valid: #59
As mentioned in that PR, I use osslsigncode for viewing details about existing signature and verifying them.

[ebourg]

Thank you for the PR. Verification is long overdue, I haven't got the time to complete the work unfortunately. I wonder if a show command would be redundant with verify, or if it would be legitimate to have both.

[JohnAZoidberg]

I wonder if a show command would be redundant with verify, or if it would be legitimate to have both.

Sometimes I don't have the signing certificate and I just want to check the signer's subject and timestamp/expiration.