Add public docs for the amazon event stream format

This adds documentation for the `application/vnd.amazon.eventstream`
wire format as well as the common protocol semanitcs that are layered
on top of it.

It does NOT provide tests. Event stream protocol tests are in the
works, but are not yet available.

Author: JordonPhillips

docs/source-2.0/additional-specs/protocols/smithy-rpc-v2.rst

@@ -53,6 +53,8 @@ list they understand when connecting to a service. A client SHOULD assume
 that a service supports ``http/1.1`` when no ``http`` or ``eventStreamHttp``
 values are provided.
 
+Event streaming uses the :ref:`amazon-eventstream` format.
+
 The following example defines a service that uses
 ``smithy.protocols#rpcv2Cbor``.