Add support for mixing px and % column widths

[mpavlinov]

Is your feature request related to a problem? Please describe.

Right now we have the following Known limitation Currently we do not support mixing of column widths with % and px

• Add support for Column Resizing and AutoSizing of % widths. - Add support for Column Resizing and AutoSizing of % widths #7662
• Add support for Multi-Column Headers with % widths and mixed widths. - feat(igxGrid): Handle % width for MCH. #7625

1. Run sample at: https://stackblitz.com/edit/angular-butmkq
2. For the column with % width (ContactName), body cell has correct width, while header has much bigger width. This is resulting in misalignment between header and body cells.

• Update related specifications for Column Resizing and Multi-Column Headers.
• Remove known issue and document sizing with mixed widths. Section can be added here. PR -> Add information regarding resizing columns in percentages and remove obsolete one. igniteui-docfx#2045

Describe the solution you'd like

Introduce support for mixing % and px column widths

Additional context

#5201

[github-actions]

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