feat(identity): mark sign as infaillible

[drHuangMHT]

Description

A minimal working example of making Keypair::sign infaillible. Refer to #4650 for more detail.

Notes & open questions

Change checklist

• [ ] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] A changelog entry has been made in the appropriate crates