Bridge: drop subscriptions when they are no longer required (#4481)

The bridge relay is **not** using `tokio`, while `jsonrpsee` does. To
make it work together, we are spawning a separate tokio task for every
jsonrpsee subscription, which holds a subscription reference. It looks
like we are not stopping those tasks when we no longer need it and when
there are more than `1024` active subscriptions, `jsonrpsee` stops
opening new subscriptions. This PR adds an `cancel` signal that is sent
to the background task when we no longer need a subscription.