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/model/models_gen.go

@@ -49,6 +49,13 @@ type Organization struct {
 type Query struct {
 }
 
+type SchemaUpdate struct {
+	Ref               string      `json:"ref"`
+	ID                string      `json:"id"`
+	SubGraphs         []*SubGraph `json:"subGraphs"`
+	CosmoRouterConfig *string     `json:"cosmoRouterConfig,omitempty"`
+}
+
 type SubGraph struct {
 	ID        string    `json:"id"`
 	Service   string    `json:"service"`
@@ -68,6 +75,9 @@ type SubGraphs struct {
 
 func (SubGraphs) IsSupergraph() {}
 
+type Subscription struct {
+}
+
 type Unchanged struct {
 	ID              string `json:"id"`
 	MinDelaySeconds int    `json:"minDelaySeconds"`