composer-downloads-plugin and civicrm-asset-plugin need updates for composer v2

At time of writing, composer is preparing for their next major release (v1 => v2). See also:

• https://github.com/composer/composer/issues/8726
• https://github.com/composer/composer/blob/master/UPGRADE-2.0.md#for-integrators-and-plugin-authors

From what I've seen so far, there is an easy part and a hard part for updating composer-downloads-plugin:

• Easy part: The PluginInterface expects a few more methods. Just add placeholders. (Ex: https://github.com/composer/composer/blob/master/UPGRADE-2.0.md#for-integrators-and-plugin-authors)
• Hard part: The download system has been reorganized to use ReactPHP Promises, and an internal download() method has been rearranged into multiple methods (download(), prepare(), install()).
  • Note to self: think more about the relationship between Composer\Installer\InstallationManager (which takes a lot of effort to coordinate the on-going promises) and LastCall\DownloadsPlugin\Plugin::installUpdateDownloads() (which also coordinates multiple actions).