StepRelationshipMetadata in Step should require RelationshipClass from @jupiterone/data-model

[austinkelleher]

Related: #285

We recently introduced metadata in Step, which is used to auto-generate documentation. The StepRelationshipMetadata currently has a _class property with the type string. We should change that to be RelationshipClass from @jupiterone/data-model. See here: https://github.com/JupiterOne/sdk/blob/master/packages/integration-sdk-core/src/types/step.ts#L83

Old:

export interface StepRelationshipMetadata extends StepGraphObjectMetadata {
  _class: string;
  sourceType: string;
  targetType: string;
}

New:

import { RelationshipClass } from '@jupiterone/data-model';

export interface StepRelationshipMetadata extends StepGraphObjectMetadata {
  _class: RelationshipClass;
  sourceType: string;
  targetType: string;
}