Fix the `storage_size`/`state_getStorageSize` RPC call (#13154)

* Have `KeyIterator` clone the `prefix` it receives

* Stream keys in `storage_size` RPC and add a runtime limit

* Update client/rpc/Cargo.toml

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update client/rpc/src/state/utils.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Rename the types to signify that the cancellation is due to a timeout

* Move the test into a `mod tests`

* Add a comment regarding `biased` in `tokio::select`

* Make the `clone` explicit when calling `KeyIterator::{new, new_child}`

Co-authored-by: Bastian Köcher <git@kchr.de>