refactor GetProof func and add some missing unit tests

[BeniaminDrasovean]

Reasoning behind the pull request

• Fix some TODOs

Proposed changes

• Refactor GetProof() trie method to avoid a duplicated encode operation
• Add unit tests for the trie rootManager

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?