Implement file signing verification

[saschagrunert]

This issue tracks the implementation part of kubernetes/release#2383

Goal is to add the signature verification logic for files to the sign package:

release-sdk/sign/sign.go

Line 98 in fc5cf4a

// TODO: unimplemented

Unit tests have to be written as well.

[palnabarun]

/assign

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/lifecycle stale

[puerco]

/remove-lifecycle stale

[matglas]

Have worked on this now for a bit. If you want you can assign it to me. I'll try to continue the effort on this one to fully make it work.

[matglas]

Update: The implementation is far but the integration tests are stuck on a part I am not able yet to find a solution for. I'm trying to investigate but any insight or help is welcome on PR #68