Refactor ProofNode to use interface

[weiihann]

Description

This PR refactors the ProofNode struct into ProofNode interface where Binary and Edge implements it.

Rationale

Throughout the codebase, I noticed a similar pattern with ProofNode, that is:

if proofNode.Binary != nil {
  // ... do something
} else if proofNode.Edge != nil {
  // ... do something
}

This suggests that a ProofNode can be either a Binary node or an Edge node, but not both simultaneously. Therefore, ProofNode should be represented as an interface. This approach makes the code more intuitive and easier to read.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 9 lines in your changes missing coverage. Please review.

Project coverage is 78.43%. Comparing base (67430de) to head (bd81c7b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
core/trie/proof.go	88.88%	7 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2176      +/-   ##
==========================================
- Coverage   78.47%   78.43%   -0.05%     
==========================================
  Files         100      100              
  Lines        9234     9240       +6     
==========================================
+ Hits         7246     7247       +1     
- Misses       1350     1354       +4     
- Partials      638      639       +1     

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