Support adding custom attributes on ECS container instances

[SoManyHs]

Currently, the ECS Cluster construct does not support adding custom attributes on container instances. These would be necessary for for specifying a memberOf placement constraint. using the cluster query language for custom attributes. https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-placement-constraints.html

[rix0rrr]

Still relevant

[allisaurus]

Looks like setting custom attributes needs to be done at registration time by setting ECS_INSTANCE_ATTRIBUTES via user data, similar to how cluster name and spot draining is configured.

Two concerns with this:

1. setting via user data means adding/changing them will require replacing the instances :/
2. you can add up to 10 attributes to an instance, and generating/interpolating that into a user data string like we're doing for cluster names seem rather clunky. May want to instead leverage whatever solution is found for Enable adding custom user data to ECS cluster #1711

[allisaurus]

Since this will be possible after #1711 is addressed, closing as a duplicate.