Sync the client with the latest API documentation - #65

This updates the client with all TruckersMP API changes documented since January 2025.

• Add the company partners endpoint (Client::company()->partners()) with the CompanyPartnership and CompanyPartnershipIndex models. (Add in VTC Partnership API Documentation API-Documentation#15, fix partners summary API-Documentation#16, fix: incorrect VTCPartnershipsIndex definition API-Documentation#21)
• Add the event slots endpoint (Client::event()->slots()) with the EventSlot and EventSlotRequester models. (feat: event slots endpoint API-Documentation#22)
• Add the event request assigned to a server (Server::getEventRequest()) with the ServerEventRequest model. (feat: add event_request object to Server schema API-Documentation#24)
• Add the required DLCs of a company (Company::getDlcs()). (Update documentation to add VTCs required DLCs API-Documentation#18)
• Add the avatar and roles of a company member (CompanyMember::getAvatar(), CompanyMember::getRoles()). (Add roles property to VTC member schema API-Documentation#17, Add member avatar and role color to vtc endpoints API-Documentation#23)
• Add the color of a company role (CompanyRole::getColor()). (Add member avatar and role color to vtc endpoints API-Documentation#23)
• Rename the player's isUpperStaff permission to isManagement and deprecate Player::isUpperStaff() in favor of Player::isManagement(). (fix: rename isUpperStaff to isManagement API-Documentation#20)
• Fix the Dlc model receiving the DLC ID and name in the swapped order in the Event model.

No changes were needed for the company attending events endpoint (TruckersMP/API-Documentation#14, TruckersMP/API-Documentation#19); the existing request already matches the documented response.

All new and changed fields are covered by unit tests with updated fixtures.