nb-select - Add support to search the list

* [ ] bug report
* [X] feature request

### Issue description
nb-select - Add support to search the list

**Current behavior:**
Large opening list is very hard to handle

**Expected behavior:**
Search input in the top of the list to filter it.

Just to put a simple **ng-content tag with select=ng-option-input** (like nb-option, nb-option-group) on the top of the list would be grate and even do the work for any customization.