perf(milestones): refactor (#8788)

# Which Problems Are Solved

Milestones used existing events from a number of aggregates. OIDC
session is one of them. We noticed in load-tests that the reduction of
the oidc_session.added event into the milestone projection is a costly
business with payload based conditionals. A milestone is reached once,
but even then we remain subscribed to the OIDC events. This requires the
projections.current_states to be updated continuously.


# How the Problems Are Solved

The milestone creation is refactored to use dedicated events instead.
The command side decides when a milestone is reached and creates the
reached event once for each milestone when required.

# Additional Changes

In order to prevent reached milestones being created twice, a migration
script is provided. When the old `projections.milestones` table exist,
the state is read from there and `v2` milestone aggregate events are
created, with the original reached and pushed dates.

# Additional Context

- Closes https://github.com/zitadel/zitadel/issues/8800

internal/command/milestone_model.go

@@ -0,0 +1,58 @@
+package command
+
+import (
+	"github.com/zitadel/zitadel/internal/eventstore"
+	"github.com/zitadel/zitadel/internal/repository/milestone"
+)
+
+type MilestonesReachedWriteModel struct {
+	eventstore.WriteModel
+	MilestonesReached
+}
+
+func NewMilestonesReachedWriteModel(instanceID string) *MilestonesReachedWriteModel {
+	return &MilestonesReachedWriteModel{
+		WriteModel: eventstore.WriteModel{
+			AggregateID: instanceID,
+			InstanceID:  instanceID,
+		},
+		MilestonesReached: MilestonesReached{
+			InstanceID: instanceID,
+		},
+	}
+}
+
+func (m *MilestonesReachedWriteModel) Query() *eventstore.SearchQueryBuilder {
+	return eventstore.NewSearchQueryBuilder(eventstore.ColumnsEvent).
+		AddQuery().
+		AggregateTypes(milestone.AggregateType).
+		AggregateIDs(m.AggregateID).
+		EventTypes(milestone.ReachedEventType, milestone.PushedEventType).
+		Builder()
+}
+
+func (m *MilestonesReachedWriteModel) Reduce() error {
+	for _, event := range m.Events {
+		if e, ok := event.(*milestone.ReachedEvent); ok {
+			m.reduceReachedEvent(e)
+		}
+	}
+	return m.WriteModel.Reduce()
+}
+
+func (m *MilestonesReachedWriteModel) reduceReachedEvent(e *milestone.ReachedEvent) {
+	switch e.MilestoneType {
+	case milestone.InstanceCreated:
+		m.InstanceCreated = true
+	case milestone.AuthenticationSucceededOnInstance:
+		m.AuthenticationSucceededOnInstance = true
+	case milestone.ProjectCreated:
+		m.ProjectCreated = true
+	case milestone.ApplicationCreated:
+		m.ApplicationCreated = true
+	case milestone.AuthenticationSucceededOnApplication:
+		m.AuthenticationSucceededOnApplication = true
+	case milestone.InstanceDeleted:
+		m.InstanceDeleted = true
+	}
+}