[Feature] All-or-nothing multi-put operation for R2 bucket

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Description

I have an application whose state contains two variables, A and B. Both are stored in R2 at different keys. For my use case, it is necessary that every time I write a new value of A, I also write a new value of B. The application would be in an illegal state if only one variable were updated without the other.

Currently, there isn't a way for me to `put` two values in a [Bucket](https://docs.rs/worker/0.6.7/worker/struct.Bucket.html) such that either a) both values are successfully written, or b) both values fail to write.

An all-or-nothing multi-put feature would be very helpful. Another way to view this is as a small subset of a transactions API.

cc @guybedford