Avoid copying byte array for ResponseBytes

Once the `ByteArrayAsyncResponseTransformer` has gathered all the response
bytes, we still need to wrap those bytes and the response object in a
`ResponseBytes` instance - but if we use `ResponseBytes.fromByteArray()`, a
whole new byte array will be allocated, which is bad for two reasons:

* While copying, the JVM heap must briefly hold both the old & new byte arrays -
  roughly speaking, doubling the memory requirements.
* Copying the bytes from one array to another takes a little bit of CPU time
  (obviously this varies: `System.arraycopy()` for 40MB of bytes takes ~2ms
  on my M1 machine).

A faster, more memory efficient alternative to `ResponseBytes.fromByteArray()`
is `ResponseBytes.fromByteArrayUnsafe()`, added to the AWS SDK in August 2020
with aws/aws-sdk-java-v2#1977 in response to
aws/aws-sdk-java-v2#1959. The 'Unsafe' in the name
is a warning to users of this method that the underlying byte array is _not_
copied, and so could be susceptible to badly-behaving code manipulating the
contents of the byte array after the `ResponseBytes` is handed to the calling
code.

If only the trusted SDK code has access to the original byte array before
`ResponseBytes` is handed over to the caller, and once the `ResponseBytes`
instance is handed over to the caller, the SDK code has no further use for
the original byte array, then it is safe to use `ResponseBytes.fromByteArrayUnsafe()`
in the trusted SDK code, and return the resulting `ResponseBytes` to the user,
saving a double-allocation of memory, and the CPU time for the copying of
bytes.

See also:

* aws/aws-sdk-java-v2#1959
* aws/aws-sdk-java-v2#1977

Author: rtyley

src/main/java/com/madgag/aws/sdk/async/responsebytes/awssdk/core/internal/async/ByteArrayAsyncResponseTransformerAlternative.java

@@ -55,7 +55,7 @@ public ByteArrayAsyncResponseTransformerAlternative(Optional<Function<ResponseT,
     @Override
     public CompletableFuture<ResponseBytes<ResponseT>> prepare() {
         cf = new CompletableFuture<>();
-        return cf.thenApply(arr -> ResponseBytes.fromByteArray(response, arr));
+        return cf.thenApply(arr -> ResponseBytes.fromByteArrayUnsafe(response, arr));
     }
 
     @Override