feat!: Remove the use of connection classes in PubSub.

[saranshdhingra]

This PR:

• Introduces the new V2 GAPIC surface for Cloud PubSub.
• Uses the newly introduced RequestHandler flow in the following classes:
• PubSubClient, Topic, Subscription, Schema and the `Snapshot.

BREAKING_CHANGE_REASON=RC-1 for the version 2 of the PubSub library.

[bshaffer]

I know we discussed going straight to V2, so that we wouldn't have to maintain two codepaths, and to be extra careful as to not break our customers. Is that still the path you'd like to take? I think this is our best option, but we'll want to mark things deprecated ASAP in V1 (such as the gapicClient options) so that we can give customers some time (generally, 3 months) before removing them in V2.

As far as I know, the "breaking changes" we're planning are for options and classes that should have been marked internal in the first place, so the "3 months" may be flexible. However, the more time we give our customers the better!

[bshaffer]

A main concern here is that all these calls should be using the PubSub V2 GAPIC clients (e.g. Google\Cloud\PubSub\V1\Client\SubscriberClient)

[bshaffer]

One important thing we still need to do here is remove all V1 GAPIC classes and all deprecated backwards compatibility files. That includes:

V1 GAPIC files

• src/V1/PublisherClient.php
• src/V1/SchemaServiceClient.php
• src/V1/SubscriberClient.php
• src/V1/PublisherGrpcClient.php
• src/V1/SchemaServiceGrpcClient.php
• src/V1/SubscriberGrpcClient.php
• src/V1/Gapic/PublisherGapicClient.php
• src/V1/Gapic/SchemaServiceGapicClient.php
• src/V1/Gapic/SubscriberGapicClient.php
• tests/Unit/V1/PublisherClientTest.php
• tests/Unit/V1/SchemaServiceClientTest.php
• tests/Unit/V1/SubscriberClientTest.php

Backwards Compatibility Files

• src/V1/BigQueryConfig_State.php
• src/V1/CloudStorageConfig_AvroConfig.php
• src/V1/CloudStorageConfig_State.php
• src/V1/CloudStorageConfig_TextConfig.php
• src/V1/PushConfig_OidcToken.php
• src/V1/Schema_Type.php
• src/V1/StreamingPullResponse_AcknowledgeConfirmation.php
• src/V1/StreamingPullResponse_ModifyAckDeadlineConfirmation.php
• src/V1/StreamingPullResponse_SubscriptionProperties.php
• src/V1/Subscription_State.php

[bshaffer]

Review of migrating.md. Mostly good with a few important changes.

[bshaffer]

I know we went over this, but this code throws an error in the new version (but doesn't in the old version):

$pubsub = new PubSubClient();
$topics = $pubsub->topics();

The error is:

PHP Fatal error:  Uncaught Google\Cloud\Core\Exception\GoogleException: No project ID was provided, and we were unable to detect a default project ID. in /Users/betterbrent/Code/tpc/vendor/google/cloud/Core/src/ClientTrait.php:231