Support for proxy

1. Jersey supports ClientProperties.PROXY_URI for Apache, Jetty & Netty client.

For HttpUrlConnector, it is not supported, though [there seems to exist a workaround](https://hiwanglong.github.io/2019/06/how-to-set-proxy-by-jersey-client/).
Tasks:

- HttpUrlConnector shall use it
- 3.1 jnh-connector shall use it
- [Test](https://github.com/eclipse-ee4j/jersey/blob/master/tests/e2e-client/src/test/java/org/glassfish/jersey/tests/e2e/client/connector/proxy/ProxyTest.java)

2. HttpUrlConnector Supports jdk properties :http.proxyHost and http.proxyPort (ignored for localhost), netty supports that too.
Tasks:

- 3.1 jnh-connector shall use it
- Apache connector shall use it.
- [Test](https://github.com/eclipse-ee4j/jersey/blob/master/tests/e2e-client/src/test/java/org/glassfish/jersey/tests/e2e/client/connector/proxy/ProxySelectorTest.java)

3. Jdk-connector supports proxy. Use it in the test, too.