feat(pubsub): split GAPIC from the pubsub handwritten package

[feywind]

This will split out the GAPIC classes into their own package (-api) and change the main handwritten library to use them instead of a built-in copy.

We're waiting on a release next week to link -api - please don't merge.

[gemini-code-assist]

Code Review

This pull request removes the auto-generated GAPIC client files and configuration JSONs, delegating them to the newly added external dependency @google-cloud/pubsub-api. However, two critical runtime issues were identified in the review: first, requiring a configuration JSON directly from the src/ directory of @google-cloud/pubsub-api in message-stream.ts will fail when published; second, using a namespace import for p-defer in subscriber.ts will cause a runtime TypeError when invoked.

[gemini-code-assist]

p-defer exports its main functionality as a default export. Importing it using import * as defer binds defer to a namespace object (e.g., { default: [Function] }), which will cause a runtime TypeError: defer is not a function when called as defer(). Use a default import instead.

Suggested change

	import * as defer from 'p-defer';
	import defer from 'p-defer';

[feywind]

The default import version causes other issues. It might be simplest to just revert this to a require if we don't like the new * version.