gh-298 Add a per-client request factory customizer

[SlobodianBohdan]

Part of #298. Stacked on top of #299 — please merge #299 first; this PR will then be
rebased onto master to leave only the customizer commit. (Until then it shows #299's
commits too.)

Adds a per-client request factory customizer, for configuration that is not exposed as
properties.

• New HttpClientCustomizer<B> functional interface — a single generic interface where B
  is the implementation-specific builder of the configured client-http-request-factory-impl:
  java.net.http.HttpClient.Builder (jdk), org.apache.hc.client5.http.impl.classic.HttpClientBuilder
  (http-components) or org.eclipse.jetty.client.HttpClient (jetty).

• New per-client request-factory-customizer-bean property naming the bean to apply.

• The bean is resolved from the ApplicationContext and applied to the underlying client
  builder just before the request factory is built, in each implementation's helper.

  Example:

  com:
  c4-soft:
  springaddons:
  rest:
  client:
  machin-client:
  http:
  request-factory-customizer-bean: machinHttpClientCustomizer

  @bean
  HttpClientCustomizer<HttpClient.Builder> machinHttpClientCustomizer() {
  return builder -> builder.version(HttpClient.Version.HTTP_1_1);
  }

README updated with a dedicated section; tests included.