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.

Inizia leggendo la specifica collegata di Material Design bottom app bar e la feature request di MDC a cui si fa riferimento. Confronta il componente richiesto con le issue separate relative a top app bar e bottom navigation. L’issue non indica file, test o un punto di ingresso per l’implementazione; i criteri di completamento non sono definiti oltre all’aggiunta del componente.

Scritto dal modello di indicizzazione a partire dal testo della issue.