API: Label events for project hooks.

Hi there,

Description

Currently it's not possible to receive events from a project hook when issues are labelled, even though there are user facing subscriptions for events.

Proposal

I propose to add a label event type for project hooks which would be called when:

• labels are created/edited/deleted
• labels are assigned/removed to issues (alternatively, these could be part of issue_events)
• [optionally] when they are assigned to issue board lists.

Links / references

Documentation blurb

For Project API - Add Project Hook

Attribute	Type	Required	Description
...	...	...	...
label_events	boolean	no	Trigger hook on label events

For Project Integration - Webhooks - Events

Label Events

Triggered when labels are created, updated or deleted as well as when they are assigned to or removed from issues. ...