new util to update connections

[fratzinger]

brought up in #34 (thanks @robbyphillips!) we need a way to recalculate abilities on connections. Something like:

//pseudocode
import assignAbilityToConnection from "feathers-casl";

const userId = context.params.user?.id;
if (!userId) { return }
const ability = createAbility(...);

assignAbilityToConnection(ability, (connection) => {
  return connection.user?.id === userId
});