A relationship definition between object types or event and objects.
@type rel_type() :: :o2o | :o2m | :m2m | :e2o
@type t() :: %Ex4pmCore.ProcessIR.Relationship{ cardinality: String.t(), id: String.t(), metadata: map(), qualifier: String.t(), source: String.t(), target: String.t(), type: rel_type() }