Request streaming for Apache HttpClient

- Added org.springframework.http.StreamingHttpOutputMessage, which
  allows for a settable request body (as opposed to an output stream).

- Added http.client.HttpComponentsStreamingClientHttpRequest, which
  implements the above mentioned interface, mapping setBody() to a
  setEntity() call on the Apache HttpClient HttpEntityEnclosingRequest.

- Added a 'bufferRequestBody' property to the
  HttpComponentsClientHttpRequestFactory. When this property is set to
  false (default is true), we return a
  HttpComponentsStreamingClientHttpRequest instead of a (request
  buffering) HttpComponentsClientHttpRequest.

Issue: SPR-10728