Make Assets Pallet's Privileged Roles `Option`

[joepetrowski]

Change the privileged asset roles from AccountId to Option<AccountId>. When the owner is set to None, only governance would be able to restore the role.

For discussion, if owner is Some, and they want to set a lower rank role from None to Some, should that require governance?

This would look similar to the PoolRoles struct in Nomination Pools.

Originally posted by @joepetrowski in paritytech/substrate#12310 (comment)

[dharjeezy]

I see this issue is not solved yet. Can i go ahead to open a PR for it?
@joepetrowski @vieira-giulia

[vieira-giulia]

@dharjeezy
Hey, yea I do have a branch for it, figuring out what this implies in the rest of the pallet and tests, just having some problems with my computer to run things.
Anyway, I remember hearing you talk you already had this change implemented for another reason in another branch so if you just want to open a PR that's fine for me

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/preventing-attacks-on-xcm-reserves/902/5

[dharjeezy]

@dharjeezy Hey, yea I do have a branch for it, figuring out what this implies in the rest of the pallet and tests, just having some problems with my computer to run things. Anyway, I remember hearing you talk you already had this change implemented for another reason in another branch so if you just want to open a PR that's fine for me

Alright @vieira-giulia i have gone ahead to open the PR

[muharem]

could be also enum, {Root, Account(who)}, which can be extended later if needed