Add variant input property for form components.

[cetincakiroglu]

New input property variant will be introduced.

    /**
     * Specifies the input variant of the component.
     * @group Props
     */
    @Input() variant: 'filled' | 'outlined' = 'outlined';

Example:

<input type="text" pInputText variant="filled"/>
<p-autoComplete variant="filled" />