feat: add Cosmo Router config generation and PubSub support

Creates a new `GenerateCosmoRouterConfig` function to build and 
serialize a Cosmo Router configuration from subgraphs. Implements 
PubSub mechanism for managing schema updates, allowing 
subscription to updates. Adds Subscription resolver and updates 
existing structures to accommodate new functionalities. This 
enhances the system's capabilities for dynamic updates and 
configuration management.

graph/pubsub.go

@@ -0,0 +1,66 @@
+package graph
+
+import (
+	"sync"
+
+	"gitlab.com/unboundsoftware/schemas/graph/model"
+)
+
+// PubSub handles publishing schema updates to subscribers
+type PubSub struct {
+	mu          sync.RWMutex
+	subscribers map[string][]chan *model.SchemaUpdate
+}
+
+func NewPubSub() *PubSub {
+	return &PubSub{
+		subscribers: make(map[string][]chan *model.SchemaUpdate),
+	}
+}
+
+// Subscribe creates a new subscription channel for a given schema ref
+func (ps *PubSub) Subscribe(ref string) chan *model.SchemaUpdate {
+	ps.mu.Lock()
+	defer ps.mu.Unlock()
+
+	ch := make(chan *model.SchemaUpdate, 10)
+	ps.subscribers[ref] = append(ps.subscribers[ref], ch)
+
+	return ch
+}
+
+// Unsubscribe removes a subscription channel
+func (ps *PubSub) Unsubscribe(ref string, ch chan *model.SchemaUpdate) {
+	ps.mu.Lock()
+	defer ps.mu.Unlock()
+
+	subs := ps.subscribers[ref]
+	for i, sub := range subs {
+		if sub == ch {
+			// Remove this subscriber
+			ps.subscribers[ref] = append(subs[:i], subs[i+1:]...)
+			close(sub)
+			break
+		}
+	}
+
+	// Clean up empty subscriber lists
+	if len(ps.subscribers[ref]) == 0 {
+		delete(ps.subscribers, ref)
+	}
+}
+
+// Publish sends a schema update to all subscribers of a given ref
+func (ps *PubSub) Publish(ref string, update *model.SchemaUpdate) {
+	ps.mu.RLock()
+	defer ps.mu.RUnlock()
+
+	for _, ch := range ps.subscribers[ref] {
+		// Non-blocking send - if subscriber is slow, skip
+		select {
+		case ch <- update:
+		default:
+			// Channel full, subscriber is too slow - skip this update
+		}
+	}
+}