Add query-parameter mapping to GET & SSE transports.

[jmalloc]

Blocked by #33 and #14

This issue is concerned with establishing a system for mapping URL query parameters to protocol buffers messages, allowing GET requests to populate fields within an RPC request message.

I don't really want to invent something here, if there's any reasonably well-known systems for mapping query parameters to structured data I would prefer to use that.

An initial implementation may choose to omit support for nested structural data, instead only supporting request messages with scalar fields.