Rollup node: Rotate multiple batcher keys in injector so that they are used evenly

What

This MR makes the injector for the batcher use keys evenly by rotating them.

Why

This is a feature requested by operators. It's a better UX to fund the signing keys equally and have them pay similar amount of fees while injecting add_messages operations from the batcher.

How

The signers are rotated in the injector by moving the ones used for the current round at the end.

Manually testing the MR

dune exec tezt/tests/main.exe -- -f sc_rollup.ml -t 'Alpha: wasm_2_0_0 - multiple keys set for batcher' -v