Add support for readonlyrootfilesystem schema

[vtgspk]

Addition of readonlyrootfilesystem schema in ecs-params https://github.com/aws/amazon-ecs-cli/blob/mainline/README.md#using-ecs-parameters

Context

As part of security hub remediations https://docs.aws.amazon.com/securityhub/latest/userguide/ecs-controls.html#ecs-5 , many customers had to make readonlyrootfilesystem as true. We are using ecs-cli in all our pipelines and readonlyrootfilesystem is always defaulted to "false" even if the previous task definition revision has "true" in it

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

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This is a 🚀 Feature Request