[FEATURE] make select-all checkbox focusable with keyboard

[santhipriyaNeppali]

Describe the bug
In the Multiselect field, the Select All checkbox is not selectable using the keyboard either by SPACE / TAB. And once the user navigates through options in multi-select there is no way to reach the search field back using the keyboard alone.

To Reproduce

1. Open Multiselect type dropdown with Select all checkbox, try to select using the only keyboard.
   You can use https://stackblitz.com/github/bithost-gmbh/ngx-mat-select-search-example as a starting point

Expected behavior

1. To make it keyboard-friendly, the select-all checkbox should be selectable and reachable using the keyboard alone.

Screenshots

Desktop (please complete the following information):

Smartphone (please complete the following information):

Additional context
Add any other context about the problem here.

[macjohnny]

@santhipriyaNeppali thanks for raising this issue. would you like to propose a solution?