feat(material): bottom-app-bar component

## Feature Description
https://material.io/design/components/app-bars-bottom.html

Mentioned in https://github.com/angular/components/issues/13964#issuecomment-441044989

Mentioned in https://github.com/angular/components/issues/408#issuecomment-433441313

### Other Information
MDC does not yet have a `mdc-bottom-app-bar`, but there is a [feature request](https://github.com/material-components/material-components-web/issues/2659) for one.

There is a separate feature request for a `mat-top-app-bar`: https://github.com/angular/components/issues/13964

This has been opened as people keep conflating Bottom App Bars with Bottom Navigation (which we are tracking separately in https://github.com/angular/components/issues/408). Now we can point them to this issue.

Comienza leyendo la especificación enlazada de Material Design bottom app bar y la solicitud de funcionalidad de MDC referenciada. Compara el componente solicitado con las issues independientes de top app bar y bottom navigation. La issue no especifica archivos, pruebas ni un punto de entrada para la implementación; los criterios de finalización no están definidos más allá de añadir el componente.

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