Support for VPC Peering to Heroku Fir Private spaces

### Required Terms

- [x] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [x] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

postgres, redis, etc.

### Tell us about what you're trying to solve. What challenges are you facing?

We need a way to connect to a redis instance running in heroku Fir private space. In order to migrate to fir, we have to be able to load data in to the database in the new fir private space.

**Workarounds don't work:**

Since our old database is in a different private space, there's no way to connect the Fir addon to the Cedar addon using --fork either.

And there's no import/export capability for the heroku KV addon afaik.