Feature: generate maven effective pom.xml as part of attestation process

[ChaosInTheCRD]

as mentioned in #98, the pom.xml file is not always full rendered. To aid this, the mvn cli has a help command called effective-pom which renders the final pom that maven uses to execute the final task it performs.

To ensure the fullness of the pom, witness could run this to ensure that it isn't missing any context from the pom.xml that it is using to create the attestation.