Make Custom envelope serializable

[alecandido]

Now that Custom is all the rage, I noticed there was a leftover related to the type of its attributes, preventing its serialization.

(To be fair, I actually discovered it by chance in the context of qiboteam/qibolab_platforms_qrc#201, only because it is used in the iqm5q parameters...)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.87%. Comparing base (5836076) to head (a6fd145).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1102   +/-   ##
=======================================
  Coverage   50.87%   50.87%           
=======================================
  Files          63       63           
  Lines        2905     2905           
=======================================
  Hits         1478     1478           
  Misses       1427     1427           

Flag	Coverage Δ
unittests	50.87% <100.00%> (ø)

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