Expand logging support for HTTP traffic

[mislav]

The central idea of this change is that GH_DEBUG environment variable is respected the same way it is in gh CLI.

• Respect the GH_DEBUG environment variable by default, but add the option to skip it;
• Add option to opt into colorized logging;
• Add option to explicitly log HTTP headers and bodies—the new default is off;
• Colorize JSON payloads when logging HTTP bodies;
• Log form-encoded payloads;
• Pretty-print GraphQL queries when logging HTTP requests.