Add filter wc_sample_button_text and change product type to sample

[urosevic]

This is simple addition to make possible to change sample button text from functions.php w/o need to hack core plugin files, and also set product type of sample item in cart to sample.

Not sure will product type impact some plugins (can be utilized in custom shipping method plugins).

Here how text on button can be changed:

add_filter( 'wc_sample_button_text', 'custom_wc_sample_button_text' );
function custom_wc_sample_button_text( $button_text ) {
    return 'Add A Free Sample To Cart';
}