Support deleting multiple R2 keys, fixes #780

[lukevalenta]

Add support for deleting multiple R2 keys at once with delete_multiple. After this change, the Rust bindings should have parity with the Javascript API for R2 bucket operations, at least: https://developers.cloudflare.com/r2/api/workers/workers-api-reference/#bucket-method-definitions.

The implementation is similar to how delete_multiple is implemented for DO storage: https://github.com/cloudflare/workers-rs/blob/main/worker-sys/src/types/durable_object/storage.rs#L36.

Local testing with worker-sandbox appears to work fine:

cd worker-sandbox
npx wrangler dev

curl -X DELETE http://localhost:8787/r2/delete
ok

[guybedford]

Thanks, will aim to do the next release in the coming days.

[lukevalenta]

Hey @guybedford, still planning to cut a new release soon?