Adding GraphQL Extensions

[janboll]

This enables accessing the Extensions field, as defined in the response
format: https://spec.graphql.org/October2021/#sec-Response-Format

Extensions can be enabled using configuration option use_extensions.
This will change the return parameters of the generated client
functions. Making it a breaking change if enabled.

Since extensions are untyped as defined in the spec, the Client will
return an interface of type map[string]interface{}.

Issue: #183

I have:

• Written a clear PR title and description (above)
• Signed the Khan Academy CLA
• Added tests covering my changes, if applicable
• Included a link to the issue fixed, if applicable
• Included documentation, for new features
• Added an entry to the changelog

[benjaminjkraft]

This looks great, thanks for writing it and for the clear commit message and documentation.

I have a couple of small comments inline; the most important thing is the changes in the API surface graphql/client.go, where we just want to be a little picky to get everything right before it's a breaking change. Everything else is just a suggestion.

Then the last thing is if you could add a changelog entry (under the "vNext" section at the top) -- or actually two; one for the new feature and one for the breaking change to the MakeRequest interface (you can crib from the similar one in v0.2.0).

[benjaminjkraft]

Great, thanks for the changes and for the contribution! I put in a few tiny tweaks to the documentation, which reminded me that this also fixes #19.