Allow altering definition of ShippingMethod plugins

Problem/Motivation

Other contrib modules should be able to alter definitions of ShippingMethods trough hook, rather then depending on the module weight.

use Drupal\commerce_currency_resolver\Plugin\Commerce\ShippingMethod\FlatRateCurrency;

/**
 * Implements hook_commerce_shipping_method_info_alter();
 */
function commerce_currency_resolver_commerce_shipping_method_info_alter(array &$definitions) {
  $definitions['flat_rate']['class'] = FlatRateCurrency::class;
}

Proposed resolution

Add alter 'commerce_shipping_method_info' to \Drupal\commerce_shipping\ShippingMethodManager

Remaining tasks

None

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet