Add logging for client request and response bodies

[bluekeyes]

I mostly want this for GraphQL, so that we can inspect the queries made by applications. It seemed easy enough to generalize, so I included the ability to log any client request or response body. Since logging this stuff can be noisy, expensive, or leak sensitive data, it is disabled by default. Individual applications can choose if they want to log all paths or only specific paths.

Putting up a draft PR while I work through the rest of this. Currently, request body logging is implemented but untested. I still need to implement response body logging.

Fixes #49.

[IainSteers]

@bluekeyes I think it may also make sense to add the RateLimit struct to the query structs so that we can retrieve the cost and remaining limit directly on the calls.

https://docs.github.com/en/graphql/overview/resource-limitations#returning-a-calls-rate-limit-status
https://github.com/shurcooL/githubv4/blob/master/example/githubv4dev/main.go#L92-L97

[bluekeyes]

Yeah, I agree rate limit information for GraphQL would be useful. I think that will be a separate change though, since it is pretty involved - need to parse and modify incoming queries then partially decode the responses to extract the information. The only real overlap it has here is that it also needs to modify the bodies of the request and response.

[bluekeyes]

In addition to the unit tests, I ran some quick manual tests using the real GraphQL client and everything seems to work.