[ECS,Fargate] [request]: Ability to make task definition secrets optional.

**Tell us about your request**
What do you want us to build?

We're currently using Parameter Store to pull secrets into our container, and it works great, however some may be very specific to a deployment and not relevant to others. Not having that value available prevents the container from starting up and it would be nice if we didn't have to specify a dummy value for an environment we're not using.

**Which service(s) is this request for?**
Fargate/ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.

We have to define dummy values for values where we may not be using them in various deployments, and it's tedious. Being able to ignore variables marked as optional (for example client specific secrets) would allow us to define everything in the task and not cause the task to fail on startup.

**Are you currently working around this issue?**
How are you currently solving this problem?
Setting dummy variables in parameter store where they're not needed.

Start by reviewing ECS/Fargate task definition handling for secrets sourced from Parameter Store. Done should mean secrets marked optional do not prevent a task from starting when their values are absent, eliminating the need for dummy Parameter Store values.

Written by the indexing model from the issue text.