Permissions: Add idempotent add function

[NunoAlexandre]

An idea for this. Since both parameters used are the same on has() and add(), and seems it can be a recurrent pattern, why not add a new method add_idempotent() (or similar) to the trait Permissions that does exactly this? The method could have a default implementation using only has() and add() so there is no implementation on the permission pallet.

Originally posted by @lemunozm in #1160 (comment)

[lemunozm]

I would also add an ensure() method, with the following implementation:

fn ensure(scope: Self::Scope, who: AccountId, role: Self::Role) -> DispatchResult {
    T::Permissions::has(pool_id, who, role)
        .then_some(())
        .ok_or(BadOrigin.into())
}

that is used in other places.

As the idempotent one, it only makes use of the Permissions trait, so it does not need any implementation from the pallet-permissions side.