fix: add GridTreeColumn header and footer renderers

[web-padawan]

Description

Unlike other custom column types, GridTreeColumn didn't support headerRenderer and footerRenderer.
This is misaligned with the corresponding web component where only renderer function is not supported.

This PR adds missing APIs so it can be probably considered a bug fix (and therefore backported to 2.3)

Type of change

• Feature / bugfix

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0462893) 98.85% compared to head (68d9b2a) 98.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
+ Coverage   98.85%   98.90%   +0.04%     
==========================================
  Files          25       26       +1     
  Lines         175      183       +8     
  Branches       20       20              
==========================================
+ Hits          173      181       +8     
  Misses          1        1              
  Partials        1        1              

Flag	Coverage Δ
unittests	98.90% <100.00%> (+0.04%)	⬆️

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[vaadin-bot]

Hi @web-padawan and @web-padawan, when i performed cherry-pick to this commit to 2.3, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick 5d7cf31
error: could not apply 5d7cf31... fix: add GridTreeColumn header and footer renderers (#194)
hint: After resolving the conflicts, mark them with
hint: "git add/rm ", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".