This commit decouples ExchangeResult from knowledge about
WiretapClientHttpRequest/Response both of which are now private to
WiretapConnector.
ExchangeResult takes ClientHttpRequest/Response instead along with
promises for the serialized request and response content in the form
of MonoProcessor<byte[]>.
This sets up the possibility for an ExchangeResult to be created
outside and independent of WebTestClient, should we decide to make its
constructor public.
Issue: SPR-16101
a982123ed5