`--diff` should not show a diff between full/short form of CFN intrinsic functions

[Lou1415926]

Today, --diff shows a diff between these two snippets:

# Short form
Subnets:
  - !Ref: PublicSubnet1

# Full form
Subnets:
  - Ref: PublicSubnet1

CFN templates generated by CDK uses full form everywhere, whereas Copilot uses the short form. Therefore, users running --diff for the first time after they applied CDK overrides will see a large amount of diff, while a majority of them are false alarm because of this.

It'd be great if --diff can tell that they are not really different.

Doc on full/short form: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-ref.html