feat: pubsub mvp

[malandis]

• Adds a top level TopicClient for publishing and subscribing to topics
• Adds an internal PubsubClient for abstracting calls to the pubsub service
• Adds an example driver examples/topic-publish.ts and examples/topic-subscribe.ts

To run the example:

cd examples
npm run topic-subscribe cacheName topicName

# in another terminal
npm run topic-publish cacheName topicName message

For example:

npm run topic-subscribe my-cache dogs

# in another terminal
npm run topic-publish my-cache dogs poodle
npm run topic-publish my-cache dogs corgi

[malandis]

In the current form, the end-user has no way to close a subscription stream.

[cprice404]

this is looking great to me. Let's get @allenheltondev to weigh in on the example syntax / callback naming. Then just a few nits to clean up and we can merge this and deal with the rest in follow-ups.

[cprice404]

you need to split the examples out to a separate PR and revert the .. in the examples package.json but other than that I'm good to go on this

[allenheltondev]

Looks good. Thank you for updating the function names to onItem and onError!