feat: Updated remote config to include domain restrictions

[benjackwhite]

Problem

We need a way to do some level of dynamic config checking

Changes

• There is only one cached config entry now - the others are built in memory from that as they dont need other api calls
• Checks the domain like before and sets the appropriate vary headers
• Add tests to ensure the api works as expected
• Validate the CDN caches based on vary header

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Does this work well for both Cloud and self-hosted?

How did you test this code?

[pauldambra]

a little thinking out loud but 👍