[CdkTable] expose _insertRow to derived classes

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

#### What is the expected behavior?
`_insertRow` is visible to inheriting classes

#### What is the current behavior?
`_insertRow` is private

#### What is the use-case or motivation for changing an existing behavior?
In `_insertRow` the context is created (`RowContext`).
The row context is very useful as a metadata storage for the row, storing information about the row that helps when we want to create features that require state.

For example, flagging if the row is currently being edited, which cell is edited. If the row is being dragged or not etc...

This is also true for a specific cell, metadata for it can be saved on the row context.

#### Is there anything else we should know?
Because `_insertRow` calls `_renderRow`, which is also private, it will require exposing `_renderRow` as well, which is dont think is such a bad thing.

But, if we want to avoid that, maybe it will be better to expose a new protected method, `_createRowContext(data: T): RowContext ` that people can extend

首先阅读 CdkTable 的 _insertRow 和 _renderRow 方法，以及 RowContext 的创建方式。确定派生类应访问现有方法，还是应访问受保护的上下文创建入口点；当所选 API 支持派生类使用行元数据，且不会使行为变得含糊不清时，即视为完成。

由索引模型根据 Issue 内容生成。