enhancement(endpoint_gateway): added support for service endpoints

[ujjwal-ibm]

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Relates OR Closes #4466

Output from acceptance testing:

=== RUN   TestAccIBMISVirtualEndpointGateway_ServiceEndpoints
--- PASS: TestAccIBMISVirtualEndpointGateway_ServiceEndpoints (110.31s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     111.882s

=== RUN   TestAccIBMISVirtualEndpointGatewayDataSource_service_endpoints
--- PASS: TestAccIBMISVirtualEndpointGatewayDataSource_service_endpoints (118.74s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     120.718s

=== RUN   TestAccIBMISVirtualEndpointGatewaysDataSource_service_endpoints
--- PASS: TestAccIBMISVirtualEndpointGatewaysDataSource_service_endpoints (119.62s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     122.987s