feat!: server-driven unsubscribes (#1240)

1. Adds new boundary messages to the protocol allowing the server to
send a GONE batch - a set of messages for the client to know that those
rows will not receive further updates after an unsubscribe call.
2. Server is the source of truth on which rows the client should have
3. Only while-online unsubscribes are possible
4. Client may reconnect at any point after sending the unsubscribe call
and the server will adjust seen rows correctly and send a GONE batch if
we don't believe the client had seen that -- see tests in
`subscriptions_test.exs`.

Missing is a public API for unsubscribes, that's in the next commit.

.changeset/large-jars-kiss.md

@@ -0,0 +1,6 @@
+---
+"@core/electric": minor
+"electric-sql": minor
+---
+
+feat: server-driven unsubscribes to allow clients to unsubscribe from shapes