[JENKINS-75461] add top button for hetero-list, e.g. "Add Parameter"

[mawinter69]

Adds the possibility in a hetero-list (and repeatableHeteroProperty) to add an additional dropdown button at the top of the list when there is at least one item.
This is useful when it is common to have several items in such a list and the items contain itself configuration.

Enable the flag for job parameters and build steps and post build actions in freestyle jobs.

fixes #16687

Testing done

Testing Add build step

1. created a freestyle project and added a step -> the top button appears
2. added another step with the top button -> step is added to the top
3. added another step with the bottom button -> step is added at the end
4. removed the last step -> the top button disappears
5. create a step that contains inside its configuration another hetero-list and added one item (used the conditional build step plugin for this)
6. removing such an inner item has no influence on the outer hetero-list button

Tested Job Parameters and works as expcted

Testing Add post-build action the position where the new item is added is identical for top and bottom button due to honorOrder

Proposed changelog entries

• add top button for hetero-list, e.g. "Add Parameter"

Proposed changelog category

/label developer,rfe,web-ui

Proposed upgrade guidelines

N/A

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Desired reviewers

@mention

Before the changes are marked as ready-for-merge:

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[timja]

(ATH test failing)

[github-actions]

Please take a moment and address the merge conflicts of your pull request. Thanks!

[github-actions]

Please take a moment and address the merge conflicts of your pull request. Thanks!

[timja]

LGTM - not manually tested