Add ExtensionList.lookupFirst convenience method.

[Vlatombe]

When introducing an extension point that is meant to allow overriding a default behaviour with another implementation, generally the caller only cares about looking up only one implementation, the one with the highest ordinal.

See JENKINS-XXXXX.

Testing done

Proposed changelog entries

• developer: A new convenience method ExtensionList.lookupFirst allows to retrieve the first implementation of an extension point

Proposed upgrade guidelines

N/A

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[jglick]

A couple of Javadoc nits, but otherwise looks useful. Did you search for other possible usages in core?

[Vlatombe]

Surprisingly, I didn't find other usages in core. I think ArtifactManagerFactory could be reworked to use this pattern, as there is a fallback mecanism to StandardArtifactManager if no artifact manager was configured for a given build.

[jglick]

jenkins/core/src/main/java/hudson/model/Run.java

Lines 1127 to 1135 in a23a8e5

	for (ArtifactManagerFactory f : ArtifactManagerConfiguration.get().getArtifactManagerFactories()) {
	ArtifactManager mgr = f.managerFor(this);
	if (mgr != null) {
	artifactManager = mgr;
	save();
	return mgr;
	}
	}
	return new StandardArtifactManager(this);

uses a different pattern actually. So does

jenkins/core/src/main/java/jenkins/console/DefaultConsoleUrlProvider.java

Lines 44 to 46 in a23a8e5

	@DataBoundConstructor
	public DefaultConsoleUrlProvider() {
	}

[jglick]

also checked https://github.com/jenkinsci/cloudbees-folder-plugin/blob/ed1b220693d41bb6e444d2274ffbae0684cc4a4b/src/main/java/com/cloudbees/hudson/plugins/folder/relocate/DefaultRelocationUI.java#L63-L72

[timja]

/label ready-for-merge

---

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!