make `from_cfn` generic to all the manifest fields

[KollaAdithya]

Currently from_cfn is supported only for selected set of manifest fields such as environment variables, secrets and security groups

But from_cfn can be made generic to support for other fields as well in both environment and workload manifests.

Discussion from gitter: https://app.gitter.im/#/room/#aws_copilot-cli:gitter.im/$XbGC8UQEjHeWOWMnGmLvRxmWnlX_tcKl5wbRHAgekXg

[ShanikaEdiriweera]

This would be super helpful!