Support setting default value

The patch works because when I get the following code:

/**
 * Implements hook_field_create_definitions().
 */
function mymodule_list_field_create_definitions_alter(array &$definitions) {
  // Define your fields here by entity type.
  $definitions['node'] = [
    'demo_field_2' => [
      'name'    => 'demo_field_2',
      'label'   => 'This is my field',
      'type'    => 'string',
      'force'   => FALSE, 
      'bundles' => [
        'page' => [
      'default_value' => ['Lorem Ipsum'],
        ],
      ],
    ],
  ];
}

I see that I have my default value

I think we can add a default value for the field info.
So, I propose the following patch.
A test code :

/**
 * Implements hook_field_create_definitions().
 */
function contrib_todo_list_field_create_definitions_alter(array &$definitions) {
  // Define your fields here by entity type.
  $definitions['node'] = [
    'demo_field' => [
      'name'    => 'demo_field',
      'label'   => 'This is my field',
      'type'    => 'string',
      'default_value' => ['This is my default value'],
      'force'   => FALSE, 
      'bundles' => [
        'page' => [
          'default_value' => ['This is a more specific value'],
        ],
        'article' => []
      ],
    ],
  ];
}

This is my result :
Default value article :

Default value page :