Feature request: additional static context to pass from subscription to event notification

When creating a subscription, it would be great to be able to provide information (separate to the `name`) that will be passed to each event that is emitted from that subscription.

So that if your application is listening to multiple subscriptions, which might contain overlapping data, you can distribute them to the right logic block. It's particularly an issue with ephemeral subscriptions, because you can't specify the `name` of the subscription to know which sub-part of your application is interested in parsing/enriching/post-filtering that data.