Support maps for whitelist_storage

[bkchr]

Right now we only support storage keys for whitelist_storage (maybe the macro already supports this, but for sure the benchmarking infrastructure doesn't support this). For a map we would insert the prefix of the map as key. In the benchmarking code we would need to start checking if keys start with the prefix and then whitelist them. This should be backwards compatible, because a prefix should be unique and not be used by anything else.

[gui1117]

If we want to include whitelisted key in the base block weight we would also need to associated the number of keys whitelisted in the prefix.

Note that for storage of the pallet, the benchmark of the pallet can explicitly whitelist all they keys it wants with add_to_whitelist.

But this enhancement is required for whitelisting storage when benchmarking other pallets.