bot bench: Allow passing feature flags

[xermicus]

pallet-revive requires the riscv feature for benchmarks. We don't want to add it to runtime-benchmarks as that will require everybody else to have the rve toolchain installed. So we'd need a way to pass flags to the bot, however this doesn't seem to be supported currently? I imagine something like this should work: #5675 (comment)

[mordamax]

Not a problem to add, though we're going to deprecate this command bot along with old benchmarks, now we have the other command bot built as GHA, and it uses frame-omni-bencher.
I suspect this still will require riscv feature to be added in your case?

[xermicus]

Yeah we need the rve toolchain and a way to set the feature flag in the omni bencher and we should be good. Thanks!

[athei]

Just always enabling the riscv feature when benchmarking is good enough. That should be easier to implement than adding another command to parse. Both would work for us.