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.

Start by reviewing AngularFire's existing Firestore wrappers and the Firestore JS SDK's Enterprise Pipeline Queries API. Confirm the supported Pipeline operations, including regex searches, then define the wrapper surface needed for Angular parity; done means the supported operations are exposed and covered by appropriate tests.

Written by the indexing model from the issue text.