allow root to use sudo

[xlc]

Allow root to use set_key and sudo_as.

Useful to replace force_transfer etc. No need to implement force_xxx for all different kinds of operations.

This will make sudo_as useable for democracy.

[shawntabrizi]

Sounds like the change you actually want to make is to create a call_as root function in the System pallet.

I don't see this PR as qualifying to remove force_transfer in balances, as we do not assume most runtimes have the Sudo pallet, so this kind of functionality would be useful for those chains.

If you added this to System, then maybe we could make such claims.

I would also say this PR is very strange. The whole point of the Sudo pallet is to gain access to the Root origin, but if you already have access for that check to pass, then why funnel your call through Sudo at all?

[xlc]

ok. I will add a call_as to system pallet.

The reason I want this is because there is not other way to call method on behalf of another signed origin. Otherwise I will need to implement different kinds of force_xxx to many our pallets.