Allow addition of subnets to a transit gateway config without incurring downtime.

[fitz-vivodyne]

Is your feature request related to a problem? Please describe.
As discussed in #708 and documented here, adding subnets to a TGW attachment requires multiple pipeline runs to delete the existing VPC attachment, then creating a new attachment with the updated subnets.

This means potentially incurring multiple hours of downtime for resources within the VPC.

Describe the feature you'd like
We should be able to add additional subnets to a VPC/TGW attachment without incurring downtime.

Additional context
n/a