ui5-card header does not have acc role that it is clickable

[pskelin]

Describe the bug
The ui5-card header has visual indication and a headerPress event, but the corresponding acc implementation does not indicate it as clickable.

To reproduce
Steps to reproduce the behavior:

1. Go to https://sap.github.io/ui5-webcomponents/playground/components/Card/
2. inpsect the card header

Expected behavior
role=button, or check which is appropriate

Screenshots
If applicable, add screenshots to help explain your problem.

Context

• UI5 Web Components version
• OS/Platform: {...}
• Browser (if relevant): {...}
• Other information: {...}

Affected components (if known)

Log output / Any errors in the console

{...}