Introduce new EventLabel type for webhook events

[armsnyder]

Description

I implemented option 2 from #1594, which replaces all Label webhook event fields with a new EventLabel type that contains the correct fields.

How it was tested

I updated the unmarshal tests for all event types that contain labels, without touching the testdata. (These tests were not previously inspecting labels.)

Notes

• This is a breaking change to the package, by changing some field types to EventLabel.
• In the EventLabel struct, I considered making ProjectID and GroupID of type *int because they are technically nullable. GitLab will populate whichever field matches the label Type. However, since this struct is never marshalled, I decided to keep them as int.

Closes #1594

[svanharmelen]

LGTM 👍🏻