refactor!: Use multihash format for private keys

[dima74]

Description

Added support for encoding/decoding private keys to multihash module, and replaced each occurence of PRIVATE_KEY_ALGORITHM/PRIVATE_KEY_PAYLOAD pair to PRIVATE_KEY in multihash format

Linked issue

Closes #4412

Benefits

Checklist

• [x] I've read CONTRIBUTING.md
• [x] I've used the standard signed-off commit format (or will squash just before merging)
• [ ] All applicable CI checks pass (or I promised to make them pass later)
• [ ] (optional) I've written unit tests for the code changes
• [ ] I replied to all comments after code review, marking all implemented changes with thumbs up

[github-actions[bot]]

@BAStos525

[0x009922]

Looking forward to merge it soon, so that I can rebase #4456 on top of it.