SYSMOAS_ — ExternalRelationship creation and deletion operations missing from ExternalRelationshipService

Similarly to ProjectUsage, ExternalRelationship inherits from Data, hence PIM level operation definitions should be consistent for both. At the moment PIM level ExternalRelationshipService group is lacking operations for ExternalRelationship creation and deletion when compared to ProjectUsageServices, hence:

1. PIM level ExternalRelationshipService group should have createExternalRelationship and deleteExternalRelationship operations added
2. REST/HTTP PSM level operation to service mappings should provide steps on how createExternalRelationship and deleteExternalRelationship are realized through the createCommit operation

Added PIM operations and related PIM -> PSM mappings for creating and deleting external relationships

This proposal introduces createExternalRelationship and deleteExternalRelationship operations under ExternalRelationshipService as helper operations and to maintain consistency with similar operations in ProjectUsageService. It also adds PIM -> PSM mappings for these 2 new operations.