Add load balancer to gateway-task

[cthain]

Changes proposed in this PR:

• Add an optional configuration to have the gateway-task module automatically create and configure a Network Load Balancer for public ingress.
• Update the gateway-task module to create the ECS service definition.

How I've tested this PR:

• Units tests
• Tested against the mesh-gateway example.

How I expect reviewers to test this PR:

:eyes:

Checklist:

• [x] Tests added
• [x] CHANGELOG entry added

[cthain]

I botched the rebased on this branch so I'm closing this PR and will open a new one for a different branch.