Is your feature request related to a problem? Please describe.
As a team developper, it is my duty to use my company graphical theme which includes custom icon for clear action. At the moment it is impossible for me to redefine the shape of the clear button.

Describe the solution you'd like
Just like the custom template feature currently available for label, option and various other parts of the ng-select component, I would like to add a custom template for the clear button.