feat(nuxt): Feature/add initialCache option in runtimeConfig.app

[TheDutchCoder]

Note: mainly looking for feedback on:

1. Is this a welcome change?
2. Did I do it right?
3. Did I update the docs appropriately?
4. Do I need to add more docs (probably)?

I don't want to waste your time, so feel free to close this if it's an unwanted addition!

🔗 Linked issue

Currently not an open issue, but a started idea/discussion. I can make a related issue, but first wanted to run this by you guys.

❓ 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

Currently, all data is fetched with { initialCache: true } as the default setting. This setting can't be changed on a configuration level, which means people have to set { initialCache: false } on every single query if they don't want caching by default, which is cumbersome.

This setting will allow people to specify a default caching strategy on a config level:

export default defineNuxtConfig({
  runtimeConfig: {
    app: {
      initialCache: false
    }
  }
})

📝 Checklist

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

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[TheDutchCoder]

This obviously doesn't work yet, I'm probably approaching the config part wrong.
Will try to make it work so the tests pass, and add a new test case if I can!

[TheDutchCoder]

Ok, it works properly now, but I'm lost with the tests. Not really sure how they work under the hood.

[pi0]

Thanks for PR. Planning to drop initialCache and disable it by default (#8885).

BTW supporting global options that change behavior might be tricky. If a Nuxt module or library depends on default behavior, it can be broken easily.