Enhanced tracking and user management with new API fields.

Streamlined engagement tracking and improved user management



## Added
- New webhook event:
`contact_center.engagement_ended`

## Changed
- New `concurrent_email_capacity` and `email_max_agent_load` response fields in these Users APIs:
  - `GET /v2/contact_center/users/{userId}`
  - `GET /v2/contact_center/users`
- New `email_max_agent_load` and `concurrent_email_capacity` request fields in this Users API:
  - `POST /v2/contact_center/users`

