add a script to regenerate the gRPC library; commit generated gRPC libraries

[devoncarew]

• add a script to regenerate the gRPC library
• commit generated gRPC libraries

Done as two commits for easier review (the 2nd commit will be large).

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[devoncarew]

@craiglabenz, if you want to review, that would be helpful. The 2nd commit is very large but all machine generated; the bulk of the hand-written code is in the first commit.

The goal here is to generate an initial version of a gRPC Firestore client, and then fix up the generator (package:protoc_plugin) enough so that the client is functional, esp. wrt the Firestore.listen.