Extension Manager is responsible to the lifecycle of all device extension implementation.

This is a reimplementation of ExtensionManager with the support of dynamic import. The business logic will remain largely the same except the RequireJS module definition, and the associated loading logic will be replaced with ES6 Promise, async and await to aid reading.

Construct an extension manager that knows about the extensions provided as the arguments during the construction of the manager. This manager will be stored by a Media instance. The player will call the manager's public functions, for example, prepare, getExtension and others, when needed during the lifecycle of the Media instance.