Let vaadin-radio-button-group set selected radio button on-load

[vicsstar]

Here's some code similar to what we have on the vaadin-radio-button repository:

<vaadin-radio-button-group value="large">
  <vaadin-radio-button value="small">Small</vaadin-radio-button>
  <vaadin-radio-button value="medium">Medium</vaadin-radio-button>
  <vaadin-radio-button value="large" checked>Large</vaadin-radio-button>
</vaadin-radio-button-group>

Scenario

If I were writing an application and wanted to select a radio-button by default as the page loads but don't care about using the radio-button checked property, I should just set the value to be selected on the enclosing radio-button-group. This will let me be able to render an arbitrary list of radio-buttons and dynamically set which is selected on page load without resorting to using if statements on each radio button to determine which one should be selected.

I believe the existing value property of the vaadin-radio-button-group can be used to accomplish this.

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