Introduce circuit kwargs to QFT class

[renatomello]

Currently it is not possible to use the QFT class with density_matrix=True.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.99%. Comparing base (d8cc501) to head (4a3b371).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1393   +/-   ##
=======================================
  Coverage   99.99%   99.99%           
=======================================
  Files          78       78           
  Lines       11192    11192           
=======================================
  Hits        11191    11191           
  Misses          1        1           

Flag	Coverage Δ
unittests	99.99% <100.00%> (ø)

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[MatteoRobbiati]

Thanks. LGTM.