fix: Updates the interface for DataTableHeader to include isSortable

[cknabe]

Closes #18172

Minor update to the DataTableHeader interface to handle a header having its own isSortable, instead of needing to be redeclared when passing into getHeaderProps.

I picked a random bug/feature to refresh my knowledge of contributing to Carbon. If this fix isn't wanted/needed, no problem.

Changelog

• DataTableHeader now has isSortable in its interface
• getHeaderProps now checks for isSortable in the header first ~ there may be backwards incompatibilities or side effects

Testing / Reviewing

• I added a minor test that verifies a column does not allow sorting. Adding testing updates for this change seems like overkill, or possibly unwarranted. We can remove the testing updates if desired.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.17%. Comparing base (a62d322) to head (cddd718).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18295      +/-   ##
==========================================
- Coverage   84.17%   84.17%   -0.01%     
==========================================
  Files         408      408              
  Lines       14434    14436       +2     
  Branches     4689     4691       +2     
==========================================
+ Hits        12150    12151       +1     
  Misses       2120     2120              
- Partials      164      165       +1     

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