Add ability to configure maxConcurrentOperations (#329)

* Add ability to configure maxConcurrentOperations

## Summary

PINCache uses a PINOperationQueue to perform async operations on a background queue, while preventing thread explosion. However, the amount is hardcoded to 10 and I'd like to be able to experiment with how much concurrency we use. Therefore, add a property to allow it to be configuration.

## Testing

Since it's just a configuration parameter, make sure the unit tests still pass.

* Only PINCache implements the property so move it there instead of the protocol

* Nudge github actions

Source/PINCache.h

@@ -42,6 +42,11 @@ PIN_SUBCLASSING_RESTRICTED
  */
 @property (readonly) NSUInteger diskByteCount;
 
+/**
+ Sets/gets the maximum number of concurrent operations when handling async requests.
+ */
+@property (nonatomic) NSUInteger maxConcurrentOperations;
+
 /**
  The underlying disk cache, see <PINDiskCache> for additional configuration and trimming options.
  */

Source/PINCache.m

@@ -389,6 +389,16 @@ - (void)removeAllObjects
     [_diskCache removeAllObjects];
 }
 
+- (NSUInteger)maxConcurrentOperations
+{
+    return _operationQueue.maxConcurrentOperations;
+}
+
+- (void)setMaxConcurrentOperations:(NSUInteger)maxOperations
+{
+    _operationQueue.maxConcurrentOperations = maxOperations;
+}
+
 @end
 
 @implementation PINCache (Deprecated)