Migrate op-alloy-flz to monorepo

The `op-alloy-flz` crate still lives at https://github.com/alloy-rs/flz. It should be moved into the monorepo so that all `op-alloy-*` crates are in the monorepo. Given the small commit history, it's fine to move it with a regular PR and no squash merging, but of course proper attribution to all origin al committers as co-authors.