fix: separate type parameters for input and output in ContextFunction

[disintegrator]

The type of context when using a ContextFunction is incorrect as of apollo-server-express@2.4.3. Both the input context AND output context had to be ExpressContext. This PR redefines ContextFunction to have two type parameters: an input context type (in this case it's ExpressContext) and output context type to be supplied by consumer.

Fixes #1593

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[apollo-cla]

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[cheapsteak]

Thanks so much for the PR @disintegrator !
I've pulled it down and tested that this is in fact a more correct typing of that function

[MichalLytek]

Yeah, I need that fix too!
I've tried to fix the TS errors but the req and res requirement is just a bug.

[disintegrator]

Hey thanks y’all. I’m just waiting to hear if there’ll be any issues signing this CLA as I did the work during work hours - I’m not clear on how CLAs work. I’ll sort out first thing tomorrow (London time)

[cheapsteak]

Hi folks!

We've just released 2.4.4 that includes the changes from #2350 and should address the issue that this PR also addresses
The main difference is that, at least for now, we won't be opening up the value of the produced context variable open to be passed in. Adding type arguments to the main ApolloServer constructor would be a fairly significant API decision, it may be something we'd need to do eventually but would require a bit more thought and discussion (e.g. should the type of Context be the first type argument?)

Please let us know if there was anything else that's covered by this PR that we missed in the 2.4.4 release, in the mean time I'll close this issue as part of cleanup

Thanks again to @disintegrator for your time in putting this PR together and especially the description, it's tremendously appreciated 🙏

[disintegrator]

@cheapsteak Ye I figured it’d be a big change when I created the PR. I’m glad to see you arrived at a less invasive solution. Will give it a go soon.