Add HasStyle interface to CookieConsent

[web-padawan]

Description

1. Add missing HasStyle interface to CookieConsent
2. Add unit tests to check the interface - example
3. Override methods to set classes on the actual element

Part of vaadin/platform#2801

See also

• HasStyle implementation in Flow
• Unit tests for HasStyle

[web-padawan]

The problem with CookieConsent is that some classes are set on the element when initialization:

Presumably, Java users would expect to be able to remove some of these, especially to control appearance:

consent.removeClassNames("cc-top");
consent.addClassName("cc-bottom");

So we would need to initialize the class list on the Java side to match the actual state of that div.

However, I don't think we can safely set classes on the vaadin-cookie-consent element in this case.
The reason is that host element is hidden with display: none so any CSS class can override it.

This is different for vaadin-overlay which uses display: none !important to hide itself.