Add subnet TGW attachments of existing VPC

[ColinL2021]

Is your feature request related to a problem? Please describe.
Currently when you want to expand the number of subnet TGW attachments of an existing VPC, you have to remove all of the TGW attachments. Then add previous and new TGW attachments. A common scenario is some AWS services require 3 AZs, if I have a VPC with two subnets in different AZs I would have to add a 3rd AZ to use the service. This process will cause a network outage to the resources in the VPC while the pipeline runs to remove and add the TGW attachments. This will take two pipeline runs.

Describe the feature you'd like

The addition of subnet TGW attachments should be independent of existing subnet TGW attachments in the existing VPC. This process should be non disruptive. Recommendation is to make adding subnet TGW attachments individual instead of grouping them together.

Additional context
Documentation reference

[ColinL2021]

Not possible to add individually as TGW attachments have to be added together.