ot::SettingsBase::Key enum

Rules for updating existing value structures. 1. Modifying existing key value fields in settings MUST only be done by appending new fields. Existing fields MUST NOT be deleted or modified in any way. 2. To support backward compatibility (rolling back to an older software version), code reading and processing key values MUST process key values that have longer length. Additionally, newer versions MUST update/maintain values in existing key value fields. 3. To support forward compatibility (rolling forward to a newer software version), code reading and processing key values MUST process key values that have shorter length. 4. New Key IDs may be defined in the future with the understanding that such key values are not backward compatible. Defines the keys of settings.