[ECS] Mark sidecar container as essential

[izeau]

Is your feature request related to a problem? Please describe.
My application requires a sidecar container which must not crash.

Describe the solution you'd like
I would like to have the opportunity to set the essential parameter to true -- it’s currently hardcoded to false for sidecar containers.

Describe alternatives you've considered
I’m considering using Waypoint but this missing feature is annoying.

Explain any additional use-cases

• Using a sidecar proxy
• Using aws-appconfig-agent sidecar
• Using the datadog-agent sidecar

Additional context
None at the time, maybe later