Background

The upstream GitLab Go client already supports parsing MilestoneWebhookEvent in ParseWebhook(...):

• Docs: https://pkg.go.dev/gitlab.com/gitlab-org/api/client-go/v2
• Source: https://gitlab.com/gitlab-org/api/client-go/-/blob/v2.4.0/event_parsing.go

However, the dispatcher in this repository does not currently support routing *gitlab.MilestoneWebhookEvent.

Problem

Although MilestoneWebhookEvent can already be parsed by the upstream client, it cannot be dispatched further in this project, which effectively makes it unsupported here.

Expected

Add dispatcher support for MilestoneWebhookEvent, including:

• listener definition
• listener registration
• a Dispatch(...) branch
• the corresponding handler function
• basic tests

Acceptance Criteria

• Dispatcher.Dispatch(...) can handle *gitlab.MilestoneWebhookEvent
• dispatching *gitlab.MilestoneWebhookEvent no longer returns ErrUnsupportedEvent
• registered listeners are invoked correctly