Create a new implementation for issue events that aligns with webhook event design

[bitwiseman]

Found by #1175 Related #1199

Issue events are similar to webhook events: there are a set of common attributes and then different events types have different additional fields.

See:

• https://docs.github.com/en/developers/webhooks-and-events/events/issue-event-types#issue-event-object-common-properties
• https://github.com/hub4j/github-api/blob/main/src/main/java/org/kohsuke/github/GHEventPayload.java#L22-L24

However, GHIssueEvent is implemented completely differently from GHEvent and GHEventPayload. We should have a design that is similar instead.

Doing this is likely either a breaking change or at very least an addition of a new API and deprecation of the current classes.