Adding AWS CFN Trait: cfnDefaultValue

[rnihithp]

Added a new trait for cfnDefaultValue.

cfnDefaultValue: Trait to indicate the property that's the default value.

Commit 1: adding trait definition for cfnDefaultValue
Commit 2: adding trait definition for cfnRootResourceID
Commit 3: defining validator for cfnRootResourceID
Commit 4: defining test for cfnDefaultValue - test passes
Commit 5: checkstyle
Commit 6: checkstyle
Commit 7: tests fixed, cfnRootResourceID trait/selector/validation updated
Commit 8: cfnRootResourceID validation fixed
Commit 9: removed cfnRootResourceID trait from PR

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rnihithp]

These requests have been addressed in subsequent commits

[mtdowling]

What is the significance of the trait though? I don't understand how it's used.

[rnihithp]

This is mainly for use in drift detection. What this trait does is indicate that the cfn property generated from the member has a default value. For ex if we have this :

@output
structure GetFooResponse {
    ....
    @cfnDefaultValue
    timeout: Integer
    ....
 }

We don't need the actual default value to be passed into the trait in this case. The main behavior we need with this trait is the ability to annotate a member as default so we know the specific members/properties that must match for drift detection.