pubsub: release v2 library

[hongalex]

The Pub/Sub Go library is in progress to receive a v2 update. Details will be provided in the migration guide of the library but the key differences are

1. The existing resource management API (admin operations) will be deprecated, and replaced with the gRPC autogenerated surface that the other packages in this repository use. This affects topic, subscription, and schema management and related API.
2. The Topic and Subscription structs will be renamed to Publisher and Subscriber respectively
3. Some settings fields will be removed (ReceiveSettings.Synchronous, PublishSettings.BufferedByteLimit, ReceiveSettings.UseLegacyFlowControl)
4. Some settings fields will be renamed (e.g. MaxExtensionPeriod -> MaxDurationPerLeaseExtension)
5. Some default settings will be changed (ReceiveSettings.NumGoroutines will now default to 1)

We plan to support the cloud.google.com/go/pubsub (henceforth known as the v1 library) for 1 year from when the v2 library is launched, to give developers proper time to migrate to the v2. We will prioritize new features in v2, but providing security and bug fixes in the v1.

The following steps will be done, and will roughly correspond to 1 PR each, with changes being staged in the pubsub-v2-trunk branch.

• Create new v2 directory and copy existing v1 surface over
• Rewrite the imports of v2 directory to point to cloud.google.com/go/pubsub/v2, including GAPIC code chore(pubsub): rewrite v2 files with new package #11292
• Remove existing handwritten admin API and associated tests from v2 library chore(pubsub): remove handwritten layer from v2 #11367
  • Remove topic admin plane (Create, Delete, List, Update, Exists)
  • Remove subscription admin plane
  • Remove snapshot admin plane
  • Remove schema admin plane
• Settings refactoring (removal, renaming, default settings changes) deprecate(pubsub/v2): remove and rename existing settings #11375
  • Remove data plane logic for synchronous mode
  • BufferedByteLimit should now only be configured by MaxOutstandingBytes
• Refactor tests that use removed handwritten admin API to use the v2 GAPIC library
  • Expose autogenerated library (to be used for admin operations) in the Pub/Sub client
• Rename Topic/Subscription to Publisher/Subscriber (no file name changes) feat(pubsub/v2): rename data plane clients #11376
  • Support referencing Publisher by either topicID or full topic path (projects/p/topics/my-topic)
  • Remove TopicInProject, SubscriptionInProject fields
• Mark v1 package as deprecated, and all operations that will be removed
• Configure GAPIC generator to start generating new v2 library
  • Remove temporary GAPIC library used for testing
• Rename publisher.go and subscriber.go file names

Internal docs link: go/pubsub-admin-generate-go