frame::benchmarking: Support custom Config trait

[bkchr]

When creating a benchmark, sometimes it requires special logic. Currently we add this logic to the main Config trait, hidden behind the runtime-benchmarks feature flag. It would be cleaner if there would be a benchmarking specific Config trait. This trait could maybe set the pallet Config trait directly as super trait and then we may not need to change that much?

CC @ggwpez @sam0x17

[ggwpez]

Pallet NFT is grouping all benchmark related things into its own trait, so it just adds one item to the Config.

Do you mean we should add a new item like #[pallet::benchmark_config] to the pallet?
Benchmarks already support where bounds, so maybe it is enough to add <T as Config>: BenchmarkHelperTrait and then have a impl pallet::BenchmarkHelperTrait for Runtime in the runtime file without any change in FRAME.

Would be a good thing to explore in a benchmarking examples pallet.

[bkchr]

Do you mean we should add a new item like #[pallet::benchmark_config] to the pallet?

Yeah something like this.

trait BenchmarkingConfig: pallet::Config {
    type Whatever;
}

In the end this is just some "cosmetic". IMO it feels "dirty" to add the benchmarking helper to the main pallet Config.

[sam0x17]

This seems reasonable to me