API works only partially when using an HTTP proxy

[xylo]

TL;DR: Please add support for HTTP proxies in all API parts using Apache HttpClient

Long version:

I tried to use the nextcloud-java-api within a company network which requires me to use an HTTP proxy to access the Nextcloud instance.

This is my sample code demonstrating that parts of the API work with an HTTP proxy an some do not work (the ones using Apache HttpClient):

System.setProperty("http.proxyHost", "proxy");
System.setProperty("http.proxyPort", "8000");
System.setProperty("https.proxyHost", "proxy");
System.setProperty("https.proxyPort", "8000");

NextcloudConnectorconnector = new org.aarboard.nextcloud.api.NextcloudConnector("my_cloud_host", true, 443, user, password);
System.out.println("connector.getServerVersion() = " + connector.getServerVersion());  <-- works fine because it does not use Apache HttpClient

User user = connector.getUser("my_user_name");  <-- fails with a timeout because Apache HttpClient doesn't automatically use the HTTP proxy settings of Java

The problem is that in ConnectorCommon.java the HttpClient is hard coded via

HTTPC_CLIENT = HttpAsyncClients.createDefault();

So this could be either replaced by a function which can be overwritten so that the user himself can set up an HttpClient with the appropriate proxy configuration or the API could read the Java http proxy settings and set the corresponding properties of the HttpClient.

[xylo]

Implemented with PR #93.