Find an alternative for the Origami gem for initiatives PDF signatures

[ahukkanen]

Is your feature request related to a problem? Please describe.
The Origami gem which Decidim uses for adding the signatures to the initiatives PDFs seems stale at the moment.

Last release was in 2017 and last commit in the beginning of 2019.

It also seems that the maintainer is no longer actively maintaining the gem according to this:
gdelugre/origami#74

This causes maintenance issues as there are some compatibility issues in that gem with newer Ruby versions.

Describe the solution you'd like
We need to find an alternative that provides the same functionality.

Once decided, migrate to that alternative.

Describe alternatives you've considered
https://github.com/gettalong/hexapdf

Additional context
This issue was noticed during the review of #8452.