feat: add return type to aleph.vm.utils.to_json

[Psycojoker]

Add missing return type type.

Related ClickUp, GitHub or Jira tickets : ALEPH-XXX

Self proofreading checklist

• The new code clear, easy to read and well commented.
• New code does not duplicate the functions of builtin or popular libraries.
• All new code is covered by relevant tests.

How to test

Just launch mypy (the CI should do it for you)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.18%. Comparing base (5518086) to head (d77713b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #704   +/-   ##
=======================================
  Coverage   62.18%   62.18%           
=======================================
  Files          69       69           
  Lines        6079     6079           
  Branches      643      643           
=======================================
  Hits         3780     3780           
  Misses       2147     2147           
  Partials      152      152           

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