Buffer streamed chunks from stream-result endpoint

[jakelazaroff]

I ran into an issue with the stream-result endpoint where a single JSON output or result can end up split between multiple "chunks" in the HTTP response stream. One easy way to reproduce this is to stream back a result with an image, which we serialize to base 64; it ends up being split between two chunks, each of which is invalid JSON.

This PR circumvents that by processing each chunk "newline segment by newline segment". There is a value that contains the current chunk, and a buffer that is assumed to contain all unfinished JSON from previous iterations. TL;DR:

1. The SDK waits to receive a chunk, which is put into value.
2. The SDK looks in value for the substring until the first newline.
3. If there is in fact a newline, it appends that substring to the buffer (if any), parses as JSON and yields it. Then it sets the buffer to an empty string and the value to the remaining text in the chunk after the substring. GOTO 2.
4. If there's no newline, it appends the substring to the buffer. Since the chunk is finished, it waits for the next chunk; GOTO 1.