Proposal to add a second AccountId argument to the check-membership pallet function

[ltfschoen]

In the check-membership recipe, it shows how loose coupling is recommended over tight coupling.

The loose coupling example code is brilliant, and I was able to plug that into my projects codebase so I could have a membership.

But when I went to try out the code using the UI, I found that out of the box it didn't suit my use-case, and I had to make a minor tweak. And I think that my use-case would likely be more common that the current boilerplate.

So at the moment, when i tried it with polkadot.js.org/apps, the only accounts that you can add/remove to the membership are for the accounts that you have imported or created in polkadot.js.org/apps, since you can only add/remove the signer.

But I wanted to be able to add other accounts to the membership (for example from my address book, or even other accounts that weren't even mine, and I think that is a more likely scenario for most use cases.

So my proposal is just to add a second T::AccountId argument to both the add_member and remove_member functions, so the code would be like:

        #[weight = 10_000]
        pub fn add_member(
            origin,
            new_member: T::AccountId,
        ) -> DispatchResult {
            let _sender = ensure_signed(origin)?;
            // let new_member = ensure_signed(origin)?;

I'd be happy to create a PR if this is considered an improvement and a PR would be welcomed. It would require both vec-set and map-set pallets to be updated, along with their tests.
Or it could be created and labelled as an opportunity for a new Substrate developer to contribute.

Here is where i made the changes in my custom pallet where i used vec-set from Substrate recipes, and added that extra argument DataHighway-DHX/node@a6dff2a