Make cache.policies public to support dynamic policy loading.

A small step towards implementing
#5750 (comment).

src/cache/inmemory/inMemoryCache.ts

@@ -42,12 +42,16 @@ export class InMemoryCache extends ApolloCache<NormalizedCacheObject> {
   protected config: InMemoryCacheConfig;
   private watches = new Set<Cache.WatchOptions>();
   private addTypename: boolean;
-  private policies: Policies;
 
   private typenameDocumentCache = new Map<DocumentNode, DocumentNode>();
   private storeReader: StoreReader;
   private storeWriter: StoreWriter;
 
+  // Dynamically imported code can augment existing typePolicies or
+  // possibleTypes by calling cache.policies.addTypePolicies or
+  // cache.policies.addPossibletypes.
+  public readonly policies: Policies;
+
   // Set this while in a transaction to prevent broadcasts...
   // don't forget to turn it back on!
   private silenceBroadcast: boolean = false;