L2: rename l2/prover/bench crate

[xqft]

bench is a bad name for this. This crate implements a binary for

1. Downloading an L1 block and state data via RPC-JSON
2. Create inputs for the ethrex-prover from the downloaded block and state
3. Invoke ethrex-prover to prove the L1 block

We should rename it ethrex-rpc-prover or something more descriptive.

Also the fact that ethrex-prover and the posterior ethrex-rpc-prover is contained in the l2 folder can be contradictory (the prover supports both L1 and L2 blocks), but that should be addressed in a different, low priority issue.