add block.

Closes #3419241

src/Plugin/Block/AccountPickerBlock.php

+<?php
+
+namespace Drupal\simple_oauth_account_picker\Plugin\Block;
+
+use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Form\FormBuilderInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides an account picker block.
+ *
+ * @Block(
+ *   id = "simple_oauth_account_picker_block",
+ *   admin_label = @Translation("Simple Oauth Account Picker Block")
+ * )
+ */
+class AccountPickerBlock extends BlockBase implements ContainerFactoryPluginInterface {
+
+  /**
+   * Get FormBuilder service.
+   *
+   * @var \Drupal\Core\Form\FormBuilderInterface
+   */
+  protected $formBuilder;
+
+  /**
+   * Load services.
+   *
+   * @param array $configuration
+   *   The configuration.
+   * @param string $plugin_id
+   *   The plugin id.
+   * @param mixed $plugin_definition
+   *   The plugin definition.
+   * @param \Drupal\Core\Form\FormBuilderInterface $formBuilder
+   *   The form builder.
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, FormBuilderInterface $formBuilder) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+    $this->formBuilder = $formBuilder;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    // Load the service required to construct this class.
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('form_builder')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    return $this->formBuilder->getForm('Drupal\simple_oauth_account_picker\Form\AccountPickerForm');
+  }
+
+}