Generate clients to go with cppgraphqlgen services

[wravery]

This PR fixes #161. It adds the clientgen utility and a common GraphQLClient.h header and graphqlclient runtime library which can be used to pre-parse/pre-validate a query and parse the response into a statically typed struct instead of handling arbitrary.

The initial motivation was to make consuming a cppgraphqlgen service easier, but the performance savings for this approach turned out to really nice. On my machine, the single-threaded throughput of the client_benchmark using a clientgen version of the same query is more than twice the throughput of benchmark which dynamically parses/validates the query and serializes the results to JSON instead of the strongly typed struct:

> benchmark 10000
Iterations: 10000
Throughput: 9184.42 requests/second
Overall (microseconds): 1088799 total, 108.88 average
Parse (microseconds): 49 median, 43 minimum, 286 maximum, 49.4173 average
Validate (microseconds): 5 median, 4 minimum, 63 maximum, 5.477 average
Resolve (microseconds): 46 median, 39 minimum, 317 maximum, 50.139 average
ToJSON (microseconds): 2 median, 1 minimum, 26 maximum, 2.0137 average

> client_benchmark 10000
Iterations: 10000
Throughput: 20755.2 requests/second
Overall (microseconds): 481806 total, 48.1806 average
Resolve (microseconds): 45 median, 35 minimum, 314 maximum, 46.1472 average
ParseServiceResponse (microseconds): 0 median, 0 minimum, 31 maximum, 0.2003 average
ParseResponse (microseconds): 1 median, 1 minimum, 30 maximum, 1.5946 average