Remove HTTPS restriction on using a proxy

[griffinsorrentino]

Currently using a proxy is restricted to Jenkins' instances hosted on HTTP sites. Sites that are hosted using SSL / HTTPS are conditionally filtered out even if the proxy params are provided.

This change removes the "http".equals(url.getProtocol()) requirement from JnlpAgentEndpointResolver.openURLConnection(...)#605 to allow HTTPS hosted sites to use proxies.

Testing done

When trying to connect via the standard agent.jar file using the following command:
java -jar agent.jar -url https://<host> -secret <secret> -name "<name>" -workDir "<workdir>" -tunnel <host>:<port>

You are presented with the following log:

INFO: Locating server among [https://<host>]
... after 30 second timeout ...
INFO: Could not locate server among [https://<host>]; waiting 10 seconds before retry
java.io.IOException: Failed to connect to https://<host>/tcpSlaveAgentListener/: Connect timed out
	at org.jenkinsci.remoting.engine.JnlpAgentEndpointResolver.resolve(JnlpAgentEndpointResolver.java:217)
	at hudson.remoting.Engine.innerRun(Engine.java:809)
	at hudson.remoting.Engine.run(Engine.java:563)
Caused by: java.net.SocketTimeoutException: Connect timed out
	at java.base/sun.nio.ch.NioSocketImpl.timedFinishConnect(NioSocketImpl.java:546)
	at java.base/sun.nio.ch.NioSocketImpl.connect(NioSocketImpl.java:592)
	at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:327)
	at java.base/java.net.Socket.connect(Socket.java:751)
	at java.base/sun.security.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:304)
	at java.base/sun.net.NetworkClient.doConnect(NetworkClient.java:178)
	at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:531)
	at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:636)
	at java.base/sun.net.www.protocol.https.HttpsClient.<init>(HttpsClient.java:264)
	at java.base/sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:377)
	at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(AbstractDelegateHttpsURLConnection.java:193)
	at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1252)
	at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1138)
	at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:179)
	at java.base/sun.net.www.protocol.https.HttpsURLConnectionImpl.connect(HttpsURLConnectionImpl.java:141)
	at org.jenkinsci.remoting.engine.JnlpAgentEndpointResolver.resolve(JnlpAgentEndpointResolver.java:214)
	... 2 more

After removing the HTTP requirement recompiling and re-running the same command, you are presented with a successful connection

INFO: Locating server among [https://<host>/]
INFO: Remoting server accepts the following protocols: [JNLP4-connect, Ping]
INFO: Remoting TCP connection tunneling is enabled. Skipping the TCP Agent Listener Port availability check
INFO: Agent discovery successful
  Agent address: <host>
  Agent port:    <port>
  Identity:      <ident>
INFO: Handshaking
INFO: Connecting to <host>:<port>
INFO: Server reports protocol JNLP4-connect-proxy not supported, skipping
INFO: Trying protocol: JNLP4-connect
INFO: Waiting for ProtocolStack to start.
INFO: Remote identity confirmed: <ident>
INFO: Connected

This change may not be possible to write a test for as it requires an active proxy on your network.

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