add transit gateway id option to the route table

[Hamza-Olol]

what

• The existing module only allows a user to associate the route table of a private subnet with a network interface or a nat gateway.
• By adding the tgw_id argument, users of the subnet module can automatically create a route table with routing between a private subnet and a transit gateway.

why

• As a best practice, a user may create an AWS account for centralized networking.
• To allow traffic to route from account B to account A (centralized networking account), a Transit Gateway is needed.
• If a user creates a subnet using this module, the tgw_id feature will allow the user to directly associate the route table in the private subnet to an existing transit gateway.
• Example architecture: ec2 -> private subnet rtb -> tgw -> private subnet rtb-> natgw -> public internet |--------AWS Account B------|-------AWS Account A----------|

References

Multi account practices