refactor: remove Default contraint in merkle tree trait

[MarcosNicolau]

Remove Default contraint in IsMerkleTreeBackend

Description

Previously, the IsMerkleTreeBackend trait required types to implement the Default trait. However, Default is no longer used in the current Merkle tree construction logic.

Requiring Default can be unnecessarily restrictive, especially in codebases where implementing it is non-trivial. This change simplifies the trait by removing the Default constraint, making it easier to implement custom backends.

Type of change

• Refactor

Checklist

• Linked to Github Issue
• Unit tests added
• This change requires new documentation.
  • Documentation has been added/updated.
• This change is an Optimization
  • Benchmarks added/run

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.06%. Comparing base (7a336ca) to head (c44a810).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1004   +/-   ##
=======================================
  Coverage   72.06%   72.06%           
=======================================
  Files         160      160           
  Lines       35145    35145           
=======================================
  Hits        25327    25327           
  Misses       9818     9818           

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