Add a "focus on first tab" option to FocusTrap in CDK

#### Bug, feature request, or proposal:
Proposal

#### What is the expected behavior?
For dialogs and other containers such as drawers, menus etc. When the dialog or container is initialized to receive focus, instead of focusing the first tabbable element or the cdkFocusInitial directive's element, do not focus any element. Instead focus the first element upon the first tab or the last element upon shift-tab.

#### What is the current behavior?
Currently when using cdkFocusRegionStart and cdkFocusRegionEnd, the first tabbable element with the focus trap region receives initial focus. The cdkFocusInitial directive allows changing which component, but there is no mechanism to have no element initially focused. There are often use cases when no initial focus is a preferred behavior (e.g. dialogs where the primary action is non-reversible such as a delete operation).

#### What is the use-case or motivation for changing an existing behavior?
To support use cases where no initial focus is an important UX concern.

#### Is there anything else we should know?
This could be low effort. It appears that the wiring already in FocusTrap could easily support this. A new directive such as 'cdkFocusInitialTab' could be added to be the focus target on initial tab, and code changes made so that initial focus to the first tabbable element is not automatically applied when that directive is used. Usage could be something like:
```
First
...
Last
```

Start with the CDK FocusTrap implementation and the cdkFocusRegionStart/cdkFocusRegionEnd and cdkFocusInitial wiring described in the issue. Trace how initial focus is selected, then define and test the proposed cdkFocusInitialTab behavior so forward tab reaches the first element and shift-tab reaches the last without initially focusing an element.

Written by the indexing model from the issue text.