feat: subscription type

[prestwich]

Adds the following types:

• RawSubscription
• SubscriptionItem<T>
• Subscription<T>

Motivation

Lay groundwork for RAII subscriptions and subscription configuration.

Solution

Add Subscription type around the raw tokio::broadcast::Receiver

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[mattsse]

separately, sooner or later we want a stream type:

https://docs.rs/tokio-stream/latest/tokio_stream/wrappers/struct.BroadcastStream.html

[prestwich]

separately, sooner or later we want a stream type

yeah, I was considering implementing these as streams, but there's an upcoming conflict between Unpin/projection and Drop impl for RAII guards I think?

[prestwich]

basically need to figure out https://docs.rs/pin-project/latest/pin_project/attr.pinned_drop.html

[mattsse]

all of these make sense to me.

having a new type is better than dealing with receiver, so we can add functions as we like