Add M31 as known field

[georgwiese]

If rebased on top of #2051, this enables M31 RISC-V proofs with the bus enabled:

cargo run -r --features plonky3 --bin powdr-rs compile riscv/tests/riscv_data/keccak-o output --field bb
cargo run -r --features plonky3 pil output/keccak.asm -o output -f --field m31 --prove-with plonky3 --linker-mode bus

[leonardoalt]

We should also change cli to accept M31 to make it less confusing, but can do that in a different PR