fix: Update IrisGridContextMenuHandler getHeaderActions return type to be more permissive

[mattrunyon]

Needed for proper TS in deephaven/deephaven-plugins#522. Otherwise the class complains it can't properly extend IrisGridContextMenuHandler.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 46.64%. Comparing base (e75e716) to head (ffe68fd).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...d/src/mousehandlers/IrisGridContextMenuHandler.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2117   +/-   ##
=======================================
  Coverage   46.64%   46.64%           
=======================================
  Files         679      679           
  Lines       38632    38632           
  Branches     9792     9792           
=======================================
  Hits        18020    18020           
  Misses      20560    20560           
  Partials       52       52           

Flag	Coverage Δ
unit	46.64% <0.00%> (ø)

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