Creates or updates AWS Transit Gateway Network Manager (TGNM) subscription credentials. On initial setup, both awsAccessKeyId and awsSecretAccessKey are required. For updates, only awsAccessKeyId is required; if awsSecretAccessKey is omitted, the previous value is retained. The secret key is encrypted before storage. Upon successful save, the system clears any AWS connection failure alarms, reserves AWS-specific BGP ASNs (64512 for AWS, 7224 for us-east-1, 9059 for eu-west-1), initializes VTI subnet pool settings, schedules AWS polling jobs, and adds all appliances to the orchestration pending list.