Add support for configuring HttpClientTransport in JettyClientHttpConnectorBuilder

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The org.springframework.boot.http.client.reactive.JettyClientHttpConnectorBuilder is very useful, but configuration
for org.eclipse.jetty.client.HttpClientTransport perhaps isn't flexible enough. Apparently there's no way to configure
HttpClientTransport for example in this manner:

    Info h1 = HttpClientConnectionFactory.HTTP11;
    Info h2 = new ClientConnectionFactoryOverHTTP2.HTTP2(new HTTP2Client(clientConnector));

    new HttpClientTransportDynamic(clientConnector, h2, h1);

If we have a requirement for such configuration, we have no choice rather than to forfeit all the benefits of
autoconfiguration for org.springframework.http.client.reactive.ClientHttpConnector and define required components
manually. In our case it's 4 components of the following types:

• org.eclipse.jetty.io.ClientConnector.ClientConnector
• org.eclipse.jetty.client.HttpClientTransport
• org.eclipse.jetty.client.HttpClient
• and finally org.springframework.http.client.reactive.ClientHttpConnector

Of course, all of them need configuration callbacks, as a result we essentially recreated an awkward copy of
Spring Boot's autoconfiguration.