Expose AsyncLocalStorage get/set methods (#18454)

Summary:
Currently, if an app uses AsyncStorage on the JS side, there is no public interface to access stored data from the native side.

In our app, written in Swift, I have written a [helper](https://gist.github.com/ejmartin504/d501abe55c28450a0e52ac39aee7b0e6) that pulls out the data. I accomplished this by reverse-engineering the code in RCTAsyncLocalStorage.m. It would have been far easier had this code been exposed to native.

I made this change locally and tested out getting the data from Swift code. This worked like a charm:

```swift
let storage = RCTAsyncLocalStorage()
let cacheKey = "test"
storage.methodQueue?.async {
    self.storage.multiGet([cacheKey]) { values in
        print(values)
    }
}
```

[IOS ][ENHANCEMENT ][RCTAsyncLocalStorage.h] - Expose AsyncLocalStorage get/set methods to native code.

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Pull Request resolved: #18454

Differential Revision: D13860333

Pulled By: cpojer

fbshipit-source-id: b33ee5bf1ec65c8291bfcb76b0d6f0df39376a7e

React/Modules/RCTAsyncLocalStorage.h

@@ -31,4 +31,11 @@
 // For clearing data when the bridge may not exist, e.g. when logging out.
 + (void)clearAllData;
 
+// Grab data from the cache. ResponseBlock result array will have an error at position 0, and an array of arrays at position 1.
+- (void)multiGet:(NSArray<NSString *> *)keys callback:(RCTResponseSenderBlock)callback;
+
+// Add multiple key value pairs to the cache.
+- (void)multiSet:(NSArray<NSArray<NSString *> *> *)kvPairs callback:(RCTResponseSenderBlock)callback;
+
+
 @end