Add AsyncRestTemplate

Added AsyncRestTemplate, the asynchronous counterpart to the
RestTemplate that was introduced in Spring 3. All methods on the
AsyncRestTemplate are similar to those found on the synchronous
RestTemplatem, except that they return Future wrappers instead of
concrete results.

To enable this, this commit introduces the AsyncClientHttpRequest and
AsyncClientHttpRequestFactory, similar to the ClientHttpRequest and
ClientHttpRequestFactory, except that ClientHttpRequest returns a
Future<ClientHttpResponse> for the execute method. Two implementations
of these interfaces are provided, one based on the HttpURLConnection
incombination with a Spring AsyncTaskExecutor and one based on Apache
HttpComponents HttpAsyncClient.

Issue: SPR-8804