Issue title: Add support for multiple containers in ECS task definition
 Issue body: ### Issue Summary:

At the moment it is possible to define only one container image with it's options when using ECS.

### Cloud Provider(s):

ECS

### Environment:

All Spinnaker versions.

### Feature Area: 

Pipeline configurations

### Description:

I'd expect to have the possibility of defining multiple container images with their settings instead of just one.

### Additional Details:

Use case here: Datadog realtime monitoring of Fargate containers. But the sidecar support in general would be very valuable.

 Comments: 
 Comment 0: This issue hasn't been updated in 45 days, so we are tagging it as 'stale'. If you want to remove this label, comment:

> @spinnakerbot remove-label stale
 Comment 1: @spinnakerbot remove-label stale
 Comment 2: Any updates on this issue?
We are facing the same issue, which makes spinnaker unusable for us at the moment.

 Comment 3: This issue hasn't been updated in 45 days, so we are tagging it as 'stale'. If you want to remove this label, comment:

> @spinnakerbot remove-label stale
 Comment 4: @spinnakerbot remove-label stale
 Comment 5: We've bumped into first performance hiccups while migrating applications from EC2 to Fargate - which means - we've now stopped the migrations to Fargate until this is fixed - as we cannot risk it.
 Comment 6: This issue hasn't been updated in 45 days, so we are tagging it as 'stale'. If you want to remove this label, comment:

> @spinnakerbot remove-label stale
 Comment 7: @spinnakerbot remove-label stale
{'source': 'GithubIssues', 'repo_name_with_owner': 'spinnaker/spinnaker', 'issue_id': 360672561, 'repo_stars': 8977.0}