[checkbox] Add required property

[paales]

We use the element for agreements on our checkout process, and a checkbox is always required to be filled in. We'd like to be able to make the field required and add an error message when the customer doesn't fill it in.

[NickdeK]

Here is a pull request to enable this: vaadin/vaadin-checkbox#85

[web-padawan]

Related request for Flow counterpart: https://github.com/vaadin/vaadin-checkbox-flow/issues/112

[web-padawan]

We could consider using vaadin-custom-field to show required indicator.

However, as mentioned at https://github.com/vaadin/vaadin-custom-field/issues/78#issuecomment-633576433, then there is another problem: custom field currently only works with elements that have validate() method. So we are kinda stuck here.

[web-padawan]

See also vaadin/vaadin-checkbox#188 (comment)

[rolfsmeds]

Acceptance Criteria: vaadin/platform#5190

[rolfsmeds]

Also see: vaadin/flow-components#3397