Files are not synced when Drupal installed in a subdirectory

When a providing site is in a subdirectory, Entity Share fails to copy physical files over to the requesting site. The associated file entities are created, but the files themselves are missing.

To reproduce:

- Have two Drupal sites (multisite) installed in subdirectories.
- Create a standard article on one site and sync it to the other.
- The image field physical file is not copied to the destination site.

This appears to be caused by the authentication request in \Drupal\entity_share_client\Service\RemoteManager::prepareClient():

    $http_client->post('/user/login', [
      'form_params' => [
        'name' => $remote->get('basic_auth_username'),
        'pass' => $remote->get('basic_auth_password'),
        'form_id' => 'user_login_form',
      ],
    ]);

If the providing exists at http://example.com/my-subdirectory, the leading slash in /user/login causes the login request to post to http://example.com/user/login, eliminating the subdirectory name. Apparently this is the way Guzzle intentionally handle absolute directory paths combined with base_uri's with subdirectories, as described at https://stackoverflow.com/questions/25268872/guzzlehttp-client-ignores-b....