Add system activity tracking and event store (#636)

This PR adds system activity tracking. 
The management service records events like 
add/remove peer,  group, rule, route, etc.

The activity events are stored in the SQLite event store
and can be queried by the HTTP API.

management/server/activity/event.go

@@ -0,0 +1,42 @@
+package activity
+
+import (
+	"time"
+)
+
+// Event represents a network/system activity event.
+type Event struct {
+	// Timestamp of the event
+	Timestamp time.Time
+	// Activity that was performed during the event
+	Activity Activity
+	// ID of the event (can be empty, meaning that it wasn't yet generated)
+	ID uint64
+	// InitiatorID is the ID of an object that initiated the event (e.g., a user)
+	InitiatorID string
+	// TargetID is the ID of an object that was effected by the event (e.g., a peer)
+	TargetID string
+	// AccountID is the ID of an account where the event happened
+	AccountID string
+	// Meta of the event, e.g. deleted peer information like name, IP, etc
+	Meta map[string]any
+}
+
+// Copy the event
+func (e *Event) Copy() *Event {
+
+	meta := make(map[string]any, len(e.Meta))
+	for key, value := range e.Meta {
+		meta[key] = value
+	}
+
+	return &Event{
+		Timestamp:   e.Timestamp,
+		Activity:    e.Activity,
+		ID:          e.ID,
+		InitiatorID: e.InitiatorID,
+		TargetID:    e.TargetID,
+		AccountID:   e.AccountID,
+		Meta:        meta,
+	}
+}