Interception extension for Java's HttpClient

This module adds a common feature of HTTP client implementations to Java 11's java.net.http.HttpClient:

• Adding request decorators, to for example add or modify request headers.
• Adding request interception, to for example collect request metrics.

The extension is straight-forward to use by simply replacing HttpClient.builder() by InterceptableHttpClient.builder(). The extended builder API is then capable of registering decorators and interceptors by the two additional build steps addDecorator and addInterceptor.