MSHADE-147: Add flag to disable jar signing verification

[gzsombor]

This is the rebased fix for https://issues.apache.org/jira/browse/MSHADE-147.
The problem is, that certain jar files has an incorrect signature, so the shade plugin couldn't even open it.
The solution for this, is a flag, which can disable this jar verification optionally.

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[gnodet]

A test would be welcomed.

[mauro-rizzi-DSP]

Hey i'm running into this trying to shade one of my projects and I think not only this should be finished and we should get the option to avoid this but we also need the logging to be more than a generic "Invalid signature file digest for Manifest main attributes". If you're going to tell me one or more of my dependencies has an invalid signature you should at least tell me which ones so I can take action over that instead of filtering out the signature files of all my dependencies as if they had no use

[Ghost-chu]

This should be merged, I'm having the same problem.

[jira-importer]

Resolve #509

[jira-importer]

Resolve #509