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
```

Comienza con la implementación de FocusTrap de CDK y la conexión de cdkFocusRegionStart/cdkFocusRegionEnd y cdkFocusInitial descrita en el issue. Traza cómo se selecciona el foco inicial y, después, define y prueba el comportamiento propuesto de cdkFocusInitialTab para que Tab hacia delante llegue al primer elemento y Mayús-Tab llegue al último sin enfocar inicialmente ningún elemento.

Escrito por el modelo de indexación a partir del texto del issue.