cmb_Meta_Box_Validate will not be used to escape output, only sanitize input
methods should be static
use esc_url_raw for url field
new escape method escapes the meta value and provides overrides via a field callback or a filter
new method, `maybe_callback`, for checking if a field has registered a callback value. `sanitization_cb` now checks that as well as the new `cmb_Meta_Box_types::esc` method