x-secrets to support JSON Key and version

[JohnPreston]

Is your feature request related to a problem? Please describe.
Fargate now supports JSON Key for secrets in secrets manager.
I want the ability to specify the JSON key to use for a specific env var.

Describe the solution you'd like

secrets:
  BOOTSTRAP_URL:
    x-secrets:
      Name: /path/to/secret
      JsonKey: BootstrapUrl
  SASL_JAAS_CONFIG:
    x-secrets:
      Name: /path/to/secret
      JsonKey: SaslJaasConfig

Describe alternatives you've considered
Inline key and version possibly.

secrets:
  BOOTSTRAP_URL:
    x-secrets:
      Name: /path/to/secret:BootstrapUrl
  SASL_JAAS_CONFIG:
    x-secrets:
      Name: /path/to/secret:SaslJaasConfig

Additional context
https://aws.amazon.com/about-aws/whats-new/2020/11/aws-fargate-for-amazon-ecs-launches-features-focused-on-configuration-and-metrics/