Overview
========
The primary purpose of this work is to fill in the gaps in
implementation for deploying a VPC link between API Gateway, and an
ECS service. My goal was to allow setting up a {proxy+} API which would
forward to a Fargate service in a private VPC.

This has been tagged as 'ecs', but also involves changes to api gateway.

Since VPC links require an NLB, the LoadBalanced{Fargate|Ecs}Service classes
have been modified to support selecting either an ALB or an NLB.

Changes
=======
On the APIGW side, `IntegrationOptions` now accepts an optional connetion
type enum, as well as a VpcLink. `VpcLink` itself is a new construct
which accepts an array of Network Load Balancers. I also added the missing
`requestParameters` prop for `Method`, to allow properly setting up a proxy
path variable.

For ECS, in my use case I wanted to use the LoadBalanced*Service constructs, however
they only supported ALB. I have pulled all of the ELBv2 related setup
into the new `LoadBalancedService` base class, and also created a base
props interface `LoadBalancedServiceProps`. This deals with the common setup
between the Fargate and ECS services, and allows the selection of ALB or NLB.
As a side-effect of this refactoring, you can also now pass a Certificate to
`LoadBalancedEcsService`.

There is a new `Method` test for the VPC link props, as well as new tests
for both `VpcLink` and `LoadBalancedFargateService`.

…ion type is set to 'internet'