[FEATURE] Enable the use of a custom template for notFoundEntries

[ruekart]

Is your feature request related to a problem? Please describe.
When there are no options found for the user input then there's just the possibility to show a not found label, but sometimes you could want to add more than text, it could be an image or a button.

Describe the solution you'd like
I would like to use ng-content to show a custom element when there are no options as with the custom clear icon.

Describe alternatives you've considered
I have considered to add an EventEmitter as pointed out in #345 but I think it could be confusing for the user to know the option to press enter to add a new element or something else.

Additional context
Here there are some not found messages with custom content

[macjohnny]

This sounds good. We could use a directive similar to https://github.com/bithost-gmbh/ngx-mat-select-search/blob/master/src/app/mat-select-search/mat-select-search-clear.directive.ts to mark the content.

Do you want to implement this?

[ruekart]

Sure, I'm working on it.

[ruekart]

I sent this PR #382

[macjohnny]

thanks for your contribution. it is released in version 4.2.0