Allow returning `Future` for functions in `op` module

[Mr-Milk]

Closes #231

[Mr-Milk]

Hi @PauBadiaM @deeenes, I've added a sample implementation—let me know if it works for you. If you're happy with the approach, I can apply the same pattern to the other functions as well.

[codecov-commenter]

Codecov Report

:x: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 94.96%. Comparing base (aeb6953) to head (848e46e).

Files with missing lines	Patch %	Lines
src/decoupler/op/_hallmark.py	94.73%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #235   +/-   ##
=======================================
  Coverage   94.95%   94.96%           
=======================================
  Files          76       76           
  Lines        3927     3933    +6     
=======================================
+ Hits         3729     3735    +6     
  Misses        198      198           

Files with missing lines	Coverage Δ
src/decoupler/op/_hallmark.py	96.15% <94.73%> (+1.15%)	:arrow_up:

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