Implement configuration schema for siteimprove.settings

Problem/Motivation

The settings for the Siteimprove plugin are not defined in a configuration schema. The configuration schema in Drupal 8 is used to describe the structure of configuration files. It is then applied to:

• Typecast configuration to ensure type consistency (see StorableConfigBase::castValue())
• Automated persistence of configuration entity properties (see ConfigEntityBase::toArray())
• Automated generation of the configuration translation user interface (see the core module)

Steps to reproduce

1. Configure Siteimprove through the settings form at /admin/config/system/siteimprove
2. Install the config inspector module and look up the siteimprove.settings at /admin/config/development/configuration/inspect

Proposed resolution

Write a configuration schema file for the siteimprove.settings.

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

None.