refactor: simplify complexity in verify() signature implementation

[eguajardo]

Description

https://axelarnetwork.atlassian.net/browse/AXE-3276

Todos

• Unit tests
• Manual tests
• Documentation
• Connect epics/issues

Steps to Test

Expected Behaviour

Other Notes

[codecov]

Codecov Report

Attention: Patch coverage is 98.90110% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.66%. Comparing base (aad015b) to head (41f331f).

Files	Patch %	Lines
contracts/multisig/src/signing.rs	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #287      +/-   ##
==========================================
+ Coverage   90.61%   90.66%   +0.04%     
==========================================
  Files         161      161              
  Lines       17620    17673      +53     
==========================================
+ Hits        15967    16023      +56     
+ Misses       1653     1650       -3     

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