Add "generic" option to the "optional" configuration for handling nullable types

[DylanRJohnston]

This is an implementation for #251, it adds a new "generic" option for the "optional" configuration, and a companion type "optional_generic_type" which is a fully qualified type with a placeholder % for the generic parameter.

I have tested this locally with a custom optional.Value[A] implementation but I'm unsure how to add tests for this without bumping the Go version from 1.16 to 1.18. I would appreciate any input on how best to achieve this. Interested in any feedback on the design before I fill out the details on documentation and changelog.

I have:

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

[DylanRJohnston]

Here's an example of it in use https://github.com/DylanRJohnston/genqlient/compare/main...DylanRJohnston:genqlient:example

[benjaminjkraft]

I love the concept, thanks very much for implementing it! I don't have time for a proper review today but one thing I see is we'll want tests for this -- there are snapshot tests already set up and you'll just have to make some way to skip these on versions below 1.18. I'm a bit busy right now but I'll try to get back with a full review within a week or so.

[benjaminjkraft]

Actually, one more thought on integration tests: I'm ok just running all the integration tests only against 1.18+, I don't think there's much we really need to test against older Go versions there. Not sure if that will be enough to make it easy though, so what I said before still applies.

[DylanRJohnston]

@benjaminjkraft I've added a snapshot test and some more documentation. Do you want the the entire integration test to be updated to use the testutil.Option or to just leave it for now? Also should the change log be updated now, or only when do you a release?

[benjaminjkraft]

Ok, I pulled the version upgrade into #257, so ~once that merges~ you should be able to just merge and avoid dealing with that!

[benjaminjkraft]

I went ahead and merged main and added the changelog. Thanks again for writing this -- I'm planning on using it in a project I'm working on!

[DylanRJohnston-FZ]

Thanks! Apologies for not pushing this over the line myself. I just got absolutely swamped at work just after putting this pull request up. Glad to see it in main though!