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:


从 MatPaginator 组件的入口点开始，检查其模板如何布局左侧和右侧区域。使用 issue 中的示例作为使用目标，添加对左侧投影自定义内容的支持，并验证现有的分页器控件仍然在右侧对齐。

由索引模型根据 Issue 内容生成。