Allow different types of fetches on a watcher

Publicly exposing the fetch() function would allow us to make other kinds of fetches besides the `fetchIgnoringCacheData` exposed by `refetch()`. Our use case is that our custom version of `InMemoryNormalizedCache` has cache expiration, and we want to be able to hit the network only if the cache has become invalid. Alternately, we could manually check the cache, but this seems cleaner.

Sources/Apollo/GraphQLQueryWatcher.swift

@@ -43,7 +43,7 @@ public final class GraphQLQueryWatcher<Query: GraphQLQuery>: Cancellable, Apollo
     fetch(cachePolicy: .fetchIgnoringCacheData)
   }
 
-  func fetch(cachePolicy: CachePolicy) {
+  public func fetch(cachePolicy: CachePolicy) {
     fetching.mutate {
       // Cancel anything already in flight before starting a new fetch
       $0?.cancel()