Subscriptions and blocking next()

[Niederb]

Currently subscriptions are blocking in the sync case. See here:

substrate-api-client/src/rpc/jsonrpsee_client/subscription.rs

Lines 30 to 31 in 4d4f198

	block_on(self.inner.next()).map(|result| result.map_err(|e| Error::Client(Box::new(e))))
	}

Check whether it is possible to "break" this blocking by calling unsubscribe() and if yes also allow the user to use this to cancel calls to next() that are stuck.
See this previous discussion: #635 (comment)

[haerdib]

With #692 this issue is now obsolete and can be closed