refactor(tree-grid): allow specify getters for node properties

[yggg]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Custom getters for node properties

[codecov]

Codecov Report

Merging #1254 into master will increase coverage by 0.02%.
The diff coverage is 90.24%.

@@            Coverage Diff             @@
##           master    #1254      +/-   ##
==========================================
+ Coverage   79.47%   79.49%   +0.02%     
==========================================
  Files         232      232              
  Lines        7020     7024       +4     
  Branches      587      587              
==========================================
+ Hits         5579     5584       +5     
  Misses       1272     1272              
+ Partials      169      168       -1

Impacted Files	Coverage Δ
...ponents/tree-grid/data-source/tree-grid.service.ts	62.5% <0%> (ø)	⬆️
.../tree-grid/data-source/tree-grid-filter.service.ts	26.08% <0%> (ø)	⬆️
...omponents/tree-grid/data-source/tree-grid.model.ts	100% <100%> (+7.69%)	⬆️
...e/components/tree-grid/tree-grid-cell.component.ts	68% <100%> (ø)	⬆️
.../theme/components/tree-grid/tree-grid.component.ts	75% <100%> (ø)	⬆️
...ts/tree-grid/data-source/tree-grid-data.service.ts	100% <100%> (ø)	⬆️
...nts/tree-grid/data-source/tree-grid-data-source.ts	86.11% <81.81%> (+0.19%)	⬆️

https://i.imgur.com/oEtgDml.png