[in-toto] Add support for Dead Simple Signing Envelope (DSSE)

[nate-double-u]

Dead Simple Signing Envelope (DSSE) is a new signature wrapper, and a proposal to make it the default for in-toto metadata has been accepted. However, there is currently no Python implementation that can be used by in-toto and other projects. The aim of this project is to fix that by writing a fully featured and well tested DSSE implementation, and using it to allow users to generate in-toto metadata using DSSE rather than the legacy signature wrapper.

Mentor(s): Aditya Sirish (@adityasaky), Lukas Pühringer (@lukpueh)

https://github.com/cncf/mentoring/blob/main/summerofcode/2022.md#add-support-for-dead-simple-signing-envelope-dsse