Add support for Firestore Enterprise Pipeline Queries

Firestore has recently launched Pipeline operations in Enterprise edition.
These powerful queries allow for complex transformations, aggregations, and
filtering that go beyond what Core operations currently support.

It would be beneficial for AngularFire to provide wrappers
for Pipeline operations to maintain parity with the Firestore JS SDK and
provide a better Angular developer experience.

In my opinion, the ability to perform searches using regex is the most important feature.

Has a date already been set for implementation?

Thanks for your work.

首先检查 AngularFire 现有的 Firestore wrapper 以及 Firestore JS SDK 的 Enterprise Pipeline Queries API。确认受支持的 Pipeline 操作（包括正则表达式搜索），然后定义实现与 Angular 对等所需的 wrapper 接口；完成的标准是已公开受支持的操作，并由适当的测试覆盖。

由索引模型根据 Issue 内容生成。