Provide the ability to customize the cells' content of the Spreadsheet. SImilar to the custom cell editors of the Kendo jQuery Spreadsheet:

https://demos.telerik.com/kendo-ui/spreadsheet/custom-editors

Hi Team,

I would like to request the toolbar data validation tool like this Kendo UI for jQuery demo to be included in the new Angular component. 

Thank you!

I have a project that uses Kendo UI for jQuery to work with spreadsheets. I want to recreate the project from scratch in Angular using the Kendo UI for Angular Spreadsheet component.

In jQuery, I am currently using `kendo.spreadsheet.defineFunction()` (docs) to define our custom functions and I would like to do the same in Angular.