Returns the JSON of the IAM resource policy associated with the specified investigation group in a string. For example, <code>{"Version":"2012-10-17","Statement":[{"Effect":"Allow","Principal":{"Service":"aiops.alarms.cloudwatch.amazonaws.com"},"Action":["aiops:CreateInvestigation","aiops:CreateInvestigationEvent"],"Resource":"","Condition":{"StringEquals":{"aws:SourceAccount":"111122223333"},"ArnLike":{"aws:SourceArn":"arn:aws:cloudwatch:us-east-1:111122223333:alarm:"}}}]}</code>.