[ECS] [request]: Support ContainerDefinition in YAML format

**Tell us about your request**
Support writing ContainerDefinition in YAML not just JSON

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently, it only supports JSON schema for defining container tasks' definition.
In CloudFormation, you use YAML, and especially JSON and YAML should be easily convertible, I thought it'd be better that you can use YAML format for ContainerDefinition.

**Are you currently working around this issue?**
There's no workarounds here.

**Additional context**
I guess I made my point clear.

**Attachments**

Start by reviewing the ECS ContainerDefinition request and the existing JSON schema support described in the issue. The work is complete when ContainerDefinition can be written in YAML as well as JSON, including the CloudFormation use case; no repository files or tests are identified in the issue.

Written by the indexing model from the issue text.