feat(MatPaginator): Allow custom content in paginator

Hey there,

#### Feature Description

Add a ng-content tag in the left part of the paginator for custom content. Users could then add additional information like this:

```

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#### Use Case

The paginator, depending on the use case, can get quite wide. All controls are aligned to the right side. On the left side, there is a lot of space, which might be used for other information. E.g, imagine a table that lets the user select rows. In the bottom left, one could show the number of selected items - something like this:


Start at the MatPaginator component entry point and inspect how its template lays out the left and right sections. Add support for projected custom content on the left, using the example in the issue as the usage target, and verify that the existing paginator controls continue to align on the right.

Written by the indexing model from the issue text.