feat(pg-pubsub): add option to immediately get a result on listen

[ol-teuto]

Description

Motivation:

Pretty much the same as #612

subscription MySubscription {
  listen(topic: "user:2", immediate: true) {
    relatedNodeId
    query {
      user(id: "2") {
        email
      }
    }
  }
}

This allows to get a live view of the user email with id 2. Without the option to immediately trigger the query, this would depend on something triggering a NOTIFY first.

This is a non breaking change that adds a feature, it's off by default.

It seems like the tests only check the generated schema, so I only updated that test and didn't add a new one

Regarding documentation, should I open a PR on https://github.com/graphile/graphile.github.io before this is merged?

Performance impact

This is optional and disabled by default, thus there is no performance impact

Security impact

I can't think of a scenario where this impacts security

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

[benjie]

Seems like a reasonable feature to add 👍

[benjie]

The word immediate here is a little confusing - without reading the docs (which don't appear in Explorer, for example), it feels like you should set it because you want to immediately listen ("Why wouldn't I want to listen immediately?!").

We need a name for this that more intuitively encompasses what it does. I'm not super happy with initialEvent because it feels like rather than being a boolean it should be an input object to specify the initial event (I do not want to do this!). But includeInitialEvent/triggerInitialEvent/triggerEventOnStart/withStartEvent/withInitialEvent/eventOnInit/etc feel too wordy. And fireImmediately feels a bit unfriendly.

Having spent a good 5 minutes pacing the living room trying to think of a better name, right now initialEvent is my preferred name for this. But I'm sure there's a better one out there.

Naming things 🤦‍♂️

Suggested change

	immediate: {
	initialEvent: {

[benjie]

Oh, GitHub didn't notify me you'd pushed another commit! Sorry about that 😬