feat: add input validation for accounts and certificate requests endpoints

[gruyaume]

Description

This PR applies the same changes as in #175 to the remaining endpoints (accounts and certificate_requests). We add input validation and avoid returning 500 error messages when receiving bad requests.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules