Type issue in ldap_user

Problem/Motivation

❯ TypeError: Drupal\ldap_servers\Mapping::__construct(): Argument #1 ($id) must be of type string, int given, called in                                                                                              
  /<path>/docroot/modules/contrib/ldap/ldap_user/src/FieldProvider.php on line 152 in Drupal\ldap_servers\Mapping->__construct() (line 114 of                                             
  /<path>/docroot/modules/contrib/ldap/ldap_servers/src/Mapping.php).   

Debug Report

ldap_user.schema.yml defines ldapUserSyncMappings:mapping:drupal as a sequence and therefor the ids will be integers whereas the mapping definition expects a string.

FieldProvider.php has ```declare(strict_types=1);``` on top, so this becomes a type error.

The type change was added in this commit:

Commit fe4010b8  authored Feb 25, 2019
Initial version of split ldap_user configuration form with additional tests begun.

It seems like there was an effort to move from the config_object to config_entity, but it is unclear to me what the status of said migration is.

Proposed resolution

Add casting to the $id variable when initializing the mapping as a pragmatic fix.

Remaining tasks

This kind of looks like a refactor that wasn't quite finished, so probably there needs to be a follow-up for that.

User interface changes

None.

API changes

None.

Data model changes

None.