feat: add latestSchema query for retrieving schema updates

Implements the `latestSchema` query to fetch the latest schema 
updates for an organization. This change enhances the GraphQL API by
allowing clients to retrieve the most recent schema version and its 
associated subgraphs. The resolver performs necessary access checks, 
logs relevant information, and generates the Cosmo router configuration 
from fetched subgraph SDLs, returning structured schema update details.

graph/schema.graphqls

@@ -1,6 +1,7 @@
 type Query {
   organizations: [Organization!]! @auth(user: true)
   supergraph(ref: String!, isAfter: String): Supergraph! @auth(organization: true)
+  latestSchema(ref: String!): SchemaUpdate! @auth(organization: true)
 }
 
 type Mutation {