Create a structure for the Pre-Digest

[osrib]

• What does this PR do? Created a structure for the Pre-Digest that would be included in header digest
• Why are these changes needed? https://github.com/LimeChain/Fruzhin/issues/554

Checklist:

• [x] I have read the contributing guidelines.
• [x] My PR title matches the Conventional Commits spec.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[sonarcloud[bot]]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud