Missing implementation of filterTermAccessor in MatTableDataSource.

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

Missing implementation of filterTermAccessor in MatTableDataSource.
#### What is the expected behavior?
There should be the implementation of filterTermAccessor in MatTableDataSource so that it can be overridden.

#### What is the current behavior?
There is no implementation of filterTermAccessor in MatTableDataSource. Only comments are added for the uses of the same. I have searched the entire git repo but did not found any implementation.

#### What are the steps to reproduce?
Providing a StackBlitz reproduction is the *best* way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

#### What is the use-case or motivation for changing an existing behavior?

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

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

Commencez par le point d’entrée MatTableDataSource et examinez les commentaires existants décrivant filterTermAccessor et ses utilisations. Confirmez comment le filtrage obtient actuellement les termes, puis ajoutez l’implémentation surchargeable manquante et vérifiez que l’accessor peut être surchargé comme demandé.

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