Role Selection sub-item (0x54). PS3.7 Annex D.3.3.4.
Negotiates SCU/SCP roles for a specific SOP class. Each side can propose its role — SCU (service class user), SCP (service class provider), or both.
@type t() :: %Dimse.Pdu.RoleSelection{ scp_role: boolean() | nil, scu_role: boolean() | nil, sop_class_uid: String.t() | nil }