Added event dispatcher in columnStore to fetch rows on adding new column

[shrey27tri01]

Fixes #1120

Technical details

Added an event dispatcher to columns.ts, to tabularData.ts on adding a column, so that when a new column is added, tabularData.ts fetches the rows and the NULL values are reflected on the rows in the new column

Checklist

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• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[codecov-commenter]

Codecov Report

Merging #1256 (f085b64) into master (2e80bd9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1256   +/-   ##
=======================================
  Coverage   93.41%   93.41%           
=======================================
  Files         113      113           
  Lines        4330     4330           
=======================================
  Hits         4045     4045           
  Misses        285      285           

Flag	Coverage Δ
pytest-backend	93.41% <ø> (ø)

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[seancolsen]

Looks good. Thanks @shrey27tri01!