Support LoadBalancerV2s of type `gateway`

[achantavy]

Title: Support LoadBalancers of type "Gateway"

Description:

Describe your idea. Please be detailed. If a feature request, please
describe the desired behavior, what scenario it enables, and how it
would be used.

As seen in #1215, we do not support LoadBalancerV2s of type gateway. This causes a KeyError where cartography will crash.

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Any extra documentation required to understand the issue.
This feature is currently blocked on

• #1219 - NetworkInterfaces should use the new data model first
• #1024 - specifically, LoadBalancerV2s should use the new data model + they should use arn as their id.