feat(cache): add staleMaxAge option for caching header

[Intevel]

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
stale-while-revalidate gives you the option to revalidate the response in the background after a certain time.

Example:
{ swr: true, name: "mySWRData", magAge: 1, staleMaxAge: 30 }

Will create this cache-control headers:
cache-control: s-maxage=1, stale-while-revalidate=30

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[pi0]

I was thinking since we are keeping stale value up to maxAge anyway does it make sense to default it to opts.maxAge?

[Intevel]

Mostly you don't use the same values. I think leaving multiple options open to a developer is better.

[pi0]

I see, however, this is the actual behavior of cachify function right now. We only revalidate until maxAge reaches at least not before (always revalidate)

[Intevel]

Yes but that is not the point behind swr

[pi0]

stale-while-revalidate header alone tells browser/client always revalidate no?

[Intevel]

Yes, but with the options you can define when it should revalidate and not only when the response expired.

[pi0]

I know but we already know that data is not going to be invalidated until maxAge. But really we can improve default generated value later ;)

[Intevel]

You have convinced me.

[pi0]

Let's iterate with next PRs :)