Add support for StreamableResponseBody

[kumarshantanu]

Ring 1.6 added ring.core.protocols/StreamableResponseBody, which is helpful to implement Server-sent Events (SSE) in an adapter-independent manner. This is a feature request to add the support to Aleph.

Reference:

• https://github.com/ring-clojure/ring/blob/master/SPEC.md#15-response-maps
• https://fpsd.codes/blog/clojure-bites-sse
• https://github.com/kumarshantanu/ring-sse-middleware/blob/master/src/ring_sse_middleware/adapter/manifold.clj