Allow ConfigurableTargetInterface::getSummary() to return an array

Problem/Motivation

In #2829283: Language support in Feed import it appeared there is need for returning multiple lines in the summary. I checked how core handles this and it appears that \Drupal\Core\Field\FormatterInterface::settingsSummary() already handles this and returns an array.
The Field UI module than renders this summary in Drupal\field_ui\Form\EntityDisplayFormBase::buildFieldRow():

// Display a summary of the current plugin settings, and (if the
// summary is not empty) a button to edit them.
$summary = $plugin->settingsSummary();

// Allow other modules to alter the summary.
$this->alterSettingsSummary($summary, $plugin, $field_definition);

if (!empty($summary)) {
  $field_row['settings_summary'] = [
    '#type' => 'inline_template',
    '#template' => '<div class="field-plugin-summary">{{ summary|safe_join("<br />") }}</div>',
    '#context' => ['summary' => $summary],
    '#cell_attributes' => ['class' => ['field-plugin-summary-cell']],
  ];
}

Proposed resolution

To support multiple summary lines, let's allow ConfigurableTargetInterface::getSummary() to return an array. MappingForm::buildRow() can then use the same technique as what the Field UI module does.

This is an API change, but it makes the patch for the multilingual issue slightly simpler, so that's worth it in my opinion.

Remaining tasks

• Review
• Commit

User interface changes

None (I hope).

API changes

Interface Drupal\feeds\Plugin\Type\Target\ConfigurableTargetInterface changes: it's method ::getSummary() may now also return arrays. In fact, that becomes the recommended way.

Data model changes

None.

Patch will follow.