Cast ID as integer when saving form config settings

Problem/Motivation

The config admin form stores the organization ID + title. The field is saved as a 'number' type but config get/set seems to still save the field value as a string.

Steps to reproduce

• Install the module
• Fill out the config form and save
• Run drush cget drupal_contributions.settings
• Note that organization_id is enclosed in single quotes
• Also, running drush contribution:fetch returns "Identifier must be of type int." error

Proposed resolution

Cast the field value as an integer before saving config.