feat(SELECT): add input notClearFilteredOptions

[jurikonradi]

Feature Description

I have a mat-select in my Angular application with multiselect enabled. Inside the mat-select, there is an input that acts as a search for filtering the available options. The issue I am facing is that when I select some options, and then use the search input to filter and add more options to the selection, the previously selected options get cleared. I want the previously selected options to remain selected while I add more options using the search input.

(Description copied from this issue because it better explains my problem then I can do.)

Use Case

1. Multiselect's Form Control always shows values from filtered options only, deleting previously selected options if options have been changed. Please give us a possibility to show values from unfiltered options.
2. Please add example of multiselect with Search option in Select Examples, as coded in this example. It is very common request from business.