Provide an executor to execute futures after response streaming.

[amcgregor]

Effectively an extension providing an executor within the context (context.executor)

Backing executor should be constructed once on startup (e.g. serial executor, thread pool, process pool, distributed) or passed in, and execution may be deferred (tasks only actually submitted during the Extension.done callback; in production, or by default) by using a provided nested executor for the purpose.

[amcgregor]

Executor now provided by an extension in next, ref: 49481e3e6ff7e647ec04033784af47b7ceeff9fb as a roundabout path to integration.