Provide support to configure request interceptors in RestTestClient

[wilkinsona]

WebTestClient allows one or more ExchangeFilterFunctions to be registered. Spring REST Docs uses such a filter for its WebTestClient integration. Support for filtering with RestTestClient would allow REST Docs to offer integration with RestTestClient when it's bound to a server. Note that when RestTestClient is bound to a MockMvc instance, it should be possible to use REST Docs' existing MockMvc integration so REST Docs users aren't completely prevented from using RestTestClient with the status quo.