Add firestore data model protos

[keyonghan]

This is to prepare frontend firestore migration for Cocoon dashboard.

Part of flutter/flutter#142951.
One can view #3619 for a quick reference how we are gonna to use these protos.

This PR adds:

• commit_firestore.proto, a replacement of commit.proto for Datastore case
• task_firestore.proto, a replacement of task.proto for Datastore case
• commit_tasks_status.proto, a replacement of commit_status.proto for Datastore case

Note: *.pb.dart, *.pbenum.dart, *.pbjson.dart, *.pbserver.dart are auto generated based on the proto.

[keyonghan]

A friendly ping in case this falls off your plates:-)
Rest assured, this is an no-op change.

[ricardoamador]

Do you mean you will wait to merge this in? :)

[ricardoamador]

LGTM

[keyonghan]

Do you mean you will wait to merge this in? :)

Do you need me to wait? This is a no-op, and you do not need to rebase on top of these changes.