Use `consul-ecs envoy-entrypoint` to start Envoy for gateway-tasks

[cthain]

Changes proposed in this PR:

Use the consul-ecs envoy-entrypoint to run and monitor the envoy process in gateway-tasks. This change bings the gateway task inline with how mesh-task starts Envoy and ensures that Envoy is correctly started and monitored.

How I've tested this PR:

Local Lambda acceptance tests

How I expect reviewers to test this PR:

:eyes:

Checklist:

• [x] ~Tests added~ Not applicable
• [x] CHANGELOG entry added