types: Optionality of cache on PersistedQueryOptions.

The `cache` value is not required to be passed inside of `persistedQueries`
(the top-level object that's passed to Apollo Server's constructor options
to configure the APQ cache).  When it's omitted, it defaults to the (global)
`cache` also passed at the very top-level of the same constructor options.

This wasn't usually a problem in the past, but now that there are other
adjacent properties (like the `ttl` property introduced to control the
default TTL for APQ; #3623)
this has made it desirable to drop off the `cache` entirely and only pass
`ttl`.

packages/apollo-server-core/src/graphqlOptions.ts

@@ -68,7 +68,7 @@ export type DataSources<TContext> = {
 };
 
 export interface PersistedQueryOptions {
-  cache: KeyValueCache;
+  cache?: KeyValueCache;
   /**
    * Specified in **seconds**, this time-to-live (TTL) value limits the lifespan
    * of how long the persisted query should be cached.  To specify a desired