feat(select): add mat-select-header and mat-select-search component

[albyrock87]

This is a continuation of #7835 that fixes some bugs and adds an integrated search functionality.

Adds a mat-select-header component, which is a fixed header above the select's options. It allows for the user to put anything there: a custom text, or any focusable element (inputs, ...).
Note: This component only handles the positioning, styling, some basic focus management and exposes the panel id for a11y.

Adds a mat-select-search component, which can be placed inside mat-select-header. It renders a special input field that manages long list filtering automatically.
Note: The user can use his own custom search function. This input can also be used to handle the options filtering from outside.

Fixes #2812.

[crisbeto]

Sorry, but this really isn't the proper way to go about changes like these.

1. You took all of my work from feat(select): add mat-select-header component #7835 and are basically claiming it as your own by resubmitting it. It may be open source, but that doesn't mean that it's a free-for-all.
2. When it comes to large changes like this, it is better to open an issue where we can have a discussion about the API and implementation. We usually write a design doc for larger features as well.
3. Building on top of point two, if this had gone through the proper channels, it would've been obvious earlier that this goes against a lot of the design principles in the library. For example you're adding things to the CDK only to accommodate a case that is very specific to the select.

[albyrock87]

First of all I'm really sorry about the first point. I did non intend to "steal" your work, I just wanted to contribute to the project!
About the other two point I didn't know about those "proper things".
I will eventually open a new issue or reuse the existing one so we can discuss about the search feature.

Thank you

[willshowell]

@albyrock87 if you'd like to contribute, try working on a smaller feature! There are many listed here that already have clear expectations and won't require a design document. Be sure to avoid ones that already have PRs and are just pending merge. Another great way to contribute is to make updates to the documentation that you find to be confusing or lacking.

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