bench-bot: Add benchmark overhead command

[ggwpez]

Additionally to the benchmark pallet command we can expose benchmark overhead.
It has a simpler syntax than the pallet command, since it can only be called once per runtime:

polkadot benchmark overhead --chain=<RUNTIME>-dev --execution=wasm --wasm-execution=compiled --weight-path=runtime/<RUNTIME>/constants/src/weights/

Should work for the main runtimes, Polkadot, Kusama, Westend and Rococo.
This is a rather specific tuning, so for the smaller test-nets we don't need this IMO.
It will update two files in that folder: block_weights.rs and extrinsic_weights.rs.

For debugging you can add --max-ext-per-block=1 to make it faster.
There is a small issue (fixed paritytech/polkadot#5984) since it does not support custom copyright headers, so it will use the Substrate header 🙈.