Use UpdateSite implementation for connect and preValidate

[slide]

Different UpdateSites may need to connect to URLs differently, (e.g., authentication, etc.), this adds a method to UpdateSite which creates the URLConnection that will be used for connecting. This allows such things as a password protected UpdateSite which can add the authorization information to the connection prior to returning it. In addition the preValidate method can be overridden in UpdateSite implementations to validate the URL before trying to download it.

Testing done

No direct tests were added, but the full set of tests were run locally on the change.

Proposed changelog entries

• Developer: Allow UpdateSites to handle the connection for a URL for a download from that update site.

Proposed upgrade guidelines

N/A

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[timja]

This doesn't seem ideal ProxyConfiguration.open is deprecated:

jenkins/core/src/main/java/hudson/ProxyConfiguration.java

Line 315 in 7a7293e

public static URLConnection open(URL url) throws IOException {

You could potentially use this instead:

jenkins/core/src/main/java/hudson/ProxyConfiguration.java

Lines 426 to 432 in 7a7293e

	public static HttpRequest.Builder newHttpRequestBuilder(URI uri) {
	HttpRequest.Builder httpRequestBuilder = HttpRequest.newBuilder(uri);
	if (JenkinsJVM.isJenkinsJVM() && !UserAgentURLConnectionDecorator.DISABLED) {
	httpRequestBuilder.setHeader("User-Agent", UserAgentURLConnectionDecorator.getUserAgent());
	}
	return httpRequestBuilder;
	}

[slide]

I agree its not ideal, but the UpdateCenter.connect method returns a URLConnection. Is there a way to get a URLConnection from an HttpRequest.Builder?