Make CostPerByte for new bytecode storage configurable

[benluelo]

this is hardcoded:

wasmvm/lib_libwasmvm.go

Line 820 in 918cd59

const CostPerByte uint64 = 3 * 140_000

and as far as i can tell, this is never read (happy to be proven wrong though): https://github.com/CosmWasm/wasmd/blob/1d428c43e7ac1086d9fe97c59131dd07eb19ecf2/x/wasm/types/gas_register.go#L110

this should be a configurable parameter for chains that want to prevent against spam code uploads.