build: networking module for custom VPCs

Description

Include a networking module for a custom VPC that can be created for specific substation resources.

Motivation and Context

In order to allow end users to specify a specific VPC that resources should be deployed in rather than the default.

How Has This Been Tested?

We'll test in our dev envs.

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

[ ] My code follows the code style of this project.
[ ] My change requires a change to the documentation.
[ ] I have updated the documentation accordingly.

The contributions I made to this PR were based on testing and trying to figure out what the best user experience is. Here are the changes with some reasons why:

Refactoring into networking/vpc folder -- we'll want forward compatibility to add more networking modules in the future, like VPC endpoints
Refactoring subnets into maps -- when running Lambda in a custom VPC the recommendation is to use multiple availability zones to achieve high availability similar to the default VPC, this refactor makes that possible while still keeping the module easy to use
Removing the custom security group rule -- turns out it wasn't needed, the default one works fine and meets basic requirements (all internal resources are accessible plus egress to the Internet)

The point about VPC endpoints is important because NAT gateways are expensive. The cost of data processed in a NAT gateway is $0.045/GB versus $0.01/GB in a VPC endpoint (and endpoints become more affordable beyond 1 PB). At Brex we're processing upwards of 10 TB per day, so the cost savings with endpoints is significant. Overall I can't recommend that anyone use a NAT-enabled VPC unless absolutely necessary due to the cost. Once we add support for VPC endpoints, the cost will be much more in line with how we've built the rest of the project (meant to keep costs low).

brexhq / substation