feat: Optionally update VPC Route Tables for attached VPCs

[jmcorallo]

Description

When attaching a VPC to a Transit Gateway, you also need to update the VPC's Route Table(s) to direct traffic to the TGW. Currently this needs to be done outside of the module. With this change it is now possible to provide 2 extra values on the vpc_attachments object.

For example:

vpc_attachments = {
  some_vpc = {
    ... other values

    # optional new values
    vpc_route_table_ids  = module.some_vpc.private_route_table_ids # using https://github.com/terraform-aws-modules/terraform-aws-vpc
    tgw_destination_cidr = "0.0.0.0/0"
  }
}

Motivation and Context

Motivation: currently we are using this module and additionally defining our routes outside the module.
There's also this issue open: #17

Breaking Changes

If you have defined routes outside the module you cannot redefine them with these new parameters (you need to move them with terraform state mv or delete and recreate them).
Since the new parameters are optional there is no breaking change.

How Has This Been Tested?

In own environment, with and without the new values.

[tfhartmann]

@jmcorallo would you mind rebasing this against master to update your branch?

[jmcorallo]

@tfhartmann ready

[tfhartmann]

This looks great! Thanks for rebasing!

[antonbabenko]

v2.4.0 has been just released.

PS: @tfhartmann Could you please update the example code (in the examples folder) to show this and all other features which will be added into this module in action. Without examples, we will simply forget about this one. If updating example is not feasible, please make a note in README about this, at least.

Thank you!

[github-actions]

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