Make it possible to store and reuse generated certificates

[azasypkin]

Summary

Secutils.dev currently allows users to store certificate templates for generating unique ad-hoc private key/certificate pairs. This feature is great for quick testing but isn't suitable for more long-term usage. Ideally, users should be able to store and reuse generated private keys and certificates. The scope of this issue is to introduce a 'Certificates' utility within the 'Digital Certificates' utility bundle.

~~Blocked by: https://github.com/secutils-dev/secutils/issues/30~~