feat(eventstore): increase parallel write capabilities (#5940)

This implementation increases parallel write capabilities of the eventstore.
Please have a look at the technical advisories: [05](https://zitadel.com/docs/support/advisory/a10005) and  [06](https://zitadel.com/docs/support/advisory/a10006).
The implementation of eventstore.push is rewritten and stored events are migrated to a new table `eventstore.events2`.
If you are using cockroach: make sure that the database user of ZITADEL has `VIEWACTIVITY` grant. This is used to query events.

internal/eventstore/handler/v2/reduce.go

@@ -0,0 +1,21 @@
+package handler
+
+import "github.com/zitadel/zitadel/internal/eventstore"
+
+// EventReducer represents the required data
+// to work with events
+type EventReducer struct {
+	Event  eventstore.EventType
+	Reduce Reduce
+}
+
+// Reduce reduces the given event to a statement
+// which is used to update the projection
+type Reduce func(eventstore.Event) (*Statement, error)
+
+// EventReducer represents the required data
+// to work with aggregates
+type AggregateReducer struct {
+	Aggregate     eventstore.AggregateType
+	EventReducers []EventReducer
+}