feat: add dropdown for resource select strategies and fix empty resource strategy in pipeline syntax

[meiswjn]

This PR fixes a bug in the Pipeline Syntax and improves its UX.

fix: in /pipeline-syntax, when not specifying anything under "Strategy for resource selection", it would return an empty string in the generated snippet, which is an invalid input:

To resolve this, a null/empty check was added.

feat: add dropdown for resource select strategies
While the comments state which values are valid, it can also be parsed from the enum and can be made available via dropdown.

With these changes, it looks like this:

Testing done

I consider this to be a minor change without tests requiring adaption. If you think different, let me know.

• Tested /pipeline-syntax
• Tested /directive-generator

Proposed upgrade guidelines

N/A

Localizations

N/A

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[meiswjn]

From my understanding this is a false positive since the resource strategy is available regardless of permissions? However, I see that there is a permission check in

lockable-resources-plugin/src/main/java/org/jenkins/plugins/lockableresources/LockStep.java

Lines 172 to 177 in 4030359

	// check permission, security first
	if (item != null) {
	item.checkPermission(Item.CONFIGURE);
	} else {
	Jenkins.get().checkPermission(Jenkins.ADMINISTER);
	}

but not sure why

[mPokornyETM]

because of security. Just add the same check like in other dSomething actions and every body will be happy.

[mPokornyETM]

I think, it will be better to thrown some exceptuion here. Because, like in ouir example, empty string is still not valid. The code will do some default things, and nobody will catch the developer failure.