SSE (server-sent events) support.

[jmalloc]

Blocked by #13

Allow calling methods that use server streaming (and not client streaming) via the EventSource browser API.

• Populate the RPC input message as described in Implement support for specifying custom HTTP "GET" routes. #13
• Send each RPC output messages in its own "block", encode the protobuf message in the data: field
• Use a custom protocol buffers field option to mark a field within the RPC output message as the event ID. If this option is present, send its value in the id: field.
• Use the same option to mark a field within the RPC input message such that is populated with the value of the Last-Event-Id header
• Be aware of net/http/httputil: allow MIME parameters when detecting SSE in ReverseProxy golang/go#48427