Improve useSubscription hook types

[aleksandrlat]

useSubscription allows to omit config.subscription and config.variables if opts.skip is literally true.
I.e. subscription and variables are optional only when we explicitly pass true to skip option:
useSubscription(config, { skip: true }).

But in reality we never do it like this skip: true. We always use some variable. For example like this
useSubscription(config, { skip: !userId }).
But in this case with current typescript type if we use skip: !userId subscription and variables are required.

This PR makes config.subscription and config.variables optional whenever we provide skip option. The value of skip option can be known in runtime only.

The problem code example:

// userId is required variable in subscription
const subscription = graphql`
  subscription UserUpdates(userId: ID!) {
    userUpdates(id: $userId) {
      ...
    }
  }
`

const Component = ({ userId }: { userId?: string }) => {
  const skip = !userId

  useSubscription({ 
    subscription,

    // variables and userId are required here
    // but I don't have userId, so I have to do something like this 
    variables: {
      userId: skip ? '' : userId // or userId: userId || ''
    } 
  }, { skip })
}

With a fix I think code can be clearer:

// userId is required variable in subscription
const subscription = graphql`
  subscription UserUpdates(userId: ID!) {
    userUpdates(id: $userId) {
      ...
    }
  }
`

const Component = ({ userId }: { userId?: string }) => {
  const skip = !userId

  useSubscription({ 
    subscription,

    // we cannot make userId optional, but variables are optional now
    variables: skip ? undefined : {
      userId
    } 
  }, { skip })
}

@morrys I'm not sure what test to write for this, there is no runtime changes. Please let me know if it makes sense at all.
I made a change in our project and there was only 1 typescript error because in that place we didn't use generic param with useSubscription.

Thanks,
Aleksandr