Add a test proxy

[jhy]

This adds a local test proxy implementation and some test cases for using Jsoup.connect.proxy().

[jhy]

The Java 8's failed:

Error:  Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.11.0:testCompile (default-testCompile) on project jsoup: Compilation failure: Compilation failure: 
Error:  /home/runner/work/jsoup/jsoup/src/test/java/org/jsoup/integration/servlets/ProxyServlet.java:[38,56] cannot find symbol
Error:    symbol:   method asIterator()
Error:    location: interface java.util.Enumeration<java.lang.String>
Error:  /home/runner/work/jsoup/jsoup/src/test/java/org/jsoup/integration/servlets/ProxyServlet.java:[41,52] cannot find symbol
Error:    symbol:   method asIterator()
Error:    location: variable values of type java.util.Enumeration<java.lang.String>
Error:  -> [Help 1]

I wonder why the Animal Sniffer for v8 didn't see that?

[jhy]

(I factored most of the proxy implementation out in #2032 to use Jetty's proxy servlet. Particularly so we could bring in a tunnelling server to implement CONNECT proxying for HTTPS requests.)