Add `id` attribute to woocommerce_form_field args

This change allows developers to supply their own id attribute
when using the `woocommerce_form_field` template function.
The default ID is the escaped key, keeping the default behaviour.
Revise id, input_class and label_class escaping
Merge pull request #6208 from ragulka/form-field-id

Add `id` attribute to woocommerce_form_field args