Add ability to provide unique identifier for the Close button in a modal

[jgiardino]

Describe the issue. What is the expected and unexpected behavior?
I originally raised this question in patternfly/patternfly#3913, but feedback was that this is a react issue.

We are currently using data- attributes to uniquely identify elements in the DOM for the purpose of usage analytics. One element we are tracking is the Close button in a modal, but we aren't able to add any additional html attributes to this element. Currently, we're tracking this element using the class name pf-m-plain but this isn't ideal.

What is your product and what release version are you targeting?
No known release. At the moment, we're identifying this action via the pf class name provided, but this method prevents us from adding another .pf-m-plain button to the modal without breaking the metrics we capture.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[jgiardino]

Commenting to keep this open.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.