Add example to sign PDF documents like in Amtssignatur.

[flozzone]

This PR adds a example application which signs PDF using ASignSealQualified.

[flozzone]

I've added comments for the individual steps and added TODOs where needed from my perspective.

I'll list them here for reference:

1. A RSA based public key would be needed for PDF signatures (critical)
2. Get the signature already DER encoded (minor)
3. Verify compliance with Amtssignatur in terms of PDF signature metadata (major)
4. Add whole certificate chain (major)
5. Implement a visual signature as of Amtssignatur layout spec (major)