[Table] Multiple footers/headers with different data

#### Bug, feature request, or proposal:
Feature Request

#### What is the expected behavior?
To be able to make a second footer/header in which I can use the same columns but different data for the cells then the other footer/header

#### What is the current behavior?
Currently you can have multiple mat-footer-rows on a table but it has to have matColumnDefs for each one. IE let's say I have a second footer row that I want to go into the same columns that are setup but with different data. The only way I can see to do this is to make a new array of columns, send that into that footerDef and make the matColumnDefs for those.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-tcdbaj

#### What is the use-case or motivation for changing an existing behavior?
The use case is to have multiple footers or headers that have different data, about that column

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 6.0.9, Newest Material, etc

#### Is there anything else we should know?

Beginne damit, das Verhalten im verlinkten StackBlitz-Beispiel nachzustellen und die beteiligten Definitionen für Tabellenkopf und -fußzeile von Angular Material zu lesen. Die Aufgabe ist abgeschlossen, wenn mehrere Kopf- oder Fußzeilen dieselben Spalten wiederverwenden und dabei unterschiedliche Zellendaten bereitstellen können und das bestehende Beispiel dieses Verhalten demonstriert.

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