Added relationship between requirements

frontend/src/services/index.ts

@@ -10,3 +10,10 @@ export type { Requirement, RequirementCreateRequest, RequirementUpdateRequest }
 export { projectService } from './projectService'
 export type { Project, ProjectCreateRequest, ProjectUpdateRequest } from './projectService'
 export { validationService } from './validationService'
+export { relationshipService } from './relationshipService'
+export type { 
+  RelationshipType, 
+  RequirementLink, 
+  RequirementSearchResult,
+  RequirementLinkCreateRequest 
+} from './relationshipService'