SUP-1681: Webhook event type definition for struct literal creation

[james2791]

Feat (Webhook event types): Webhook event type definitions for allowing creation via struct literals

PR checklist:

• tests added -> (All pipeline template interaction with the REST API) -> Tests consistent
• examples of each service action (List, Get etc) added to the relevant examples/ folder -> No services altered
• CHANGELOG.md updated with pending release information

In previous version of Go-Buildkite; the types of agent, build, job webhook events were created as structs without inner field exporting; meaning that instances of these types couldn't be created in a struct literal manner. A instance of the base type was required first before adding its sub-fields:

jobFinishedEvent := buildkite.JobFinishedEvent{}
jobFinishedEvent.Pipeline = &buildkite.Pipeline{
	Name: buildkite.String("Android App"),
	... additional fields ...
}

This PR exports the inner types so that instances can be struct literal created while also keeping the above consuming model above in place - for example:

jobFinishedEvent := buildkite.JobFinishedEvent{
	JobEvent: buildkite.JobEvent{
		Pipeline: &buildkite.Pipeline{
			Name: buildkite.String("Android App"),
		},
		Build: ...
		Job: ...
	},
}

Addresses #170

[mcncl]

This change also means that the aliased types cannot have their own fields, they must use all and only the fields of their "event" types.

[mcncl]

Would require a major bump due to lack of backwards compatibility.

[jradtilbrook]

What if we just exported the inner fields so they could be accessed? I don't know if keeping them private is that useful

[james2791]

👆

I think thats a nice approach; similar to what I had implemented first without the inner field type declaration?

Would allow for creation like this:

jobFinishedEvent := buildkite.JobFinishedEvent{
	JobEvent: buildkite.JobEvent{
		Pipeline: &buildkite.Pipeline{
			Name: buildkite.String("Android App"),
		},
		Build: ...
		Job: ...
		...
	},
}

[jradtilbrook]

Yeah I think that seems okay. And that should be backwards compatible as well right?

[james2791]

Correct - same process in creating these struct instances as before; export just gives the chance to create them with the inner fields (i.e with declaring the JobEvent for any of the JobXEvent types - similar for agent/build ones). Thankfully its not a drastic type change that would need to constitute a x.0.0 release 👍