Added hook_oembed_providers_alter() to allow oEmbed provider list to be altered

Drupal 8.9.0 introduces a new hook, hook_oembed_providers_alter(), which allows modules to programmatically alter the oEmbed provider list used by Media. The provider list is a pre-cache array of oEmbed providers. This hook fires before \Drupal\media\OEmbed\Provider objects are generated from the provider list.

function hook_oembed_providers_alter(array &$providers) {
  // Add a custom provider.
  $providers[] = [
    'provider_name' => 'Custom Provider',
    'provider_url' => 'http://custom-provider.example.com',
    'endpoints' => [
      [
        'schemes' => [
          'http://custom-provider.example.com/id/*',
          'https://custom-provider.example.com/id/*',
        ],
        'url' => 'https://custom-provider.example.com/api/v2/oembed/',
        'discovery' => 'true',
      ],
    ],
  ];
}

This hook is useful in instances where a developer needs to add a provider that is not listed by http://oembed.com/providers.json. A developer also may use it to remove providers listed by http://oembed.com/providers.json.