move the header parts in a taglib

[mawinter69]

the taglib allows plugins to make use of the searchbox and the login stuff while easily being able to modify all the other aspects of the header.

Testing done

Manually verified that the header is not modified. Also in the downstream PR using the taglib works without problems.

see jenkinsci/customizable-header-plugin#101

Proposed changelog entries

• Provide header parts as a tag library.

Proposed upgrade guidelines

N/A

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[daniel-beck]

This is now public API. Are we confident a redesign doesn't just randomly change this anytime soon?

[mawinter69]

Any substantial change to the header would require a change of the header version in the PartialHeader so that plugins need to react. I might need to stick to the PartialHeader in the PR of my plugin (which is so far the only implementation of a header) https://github.com/jenkinsci/customizable-header-plugin/blob/295683015391e800be41c568b42ac80f6fd7c61b/src/main/java/io/jenkins/plugins/customizable_header/headers/LogoHeader.java#L9

[timja]

Makes sense, downstream changes look good

[timja]

/label ready-for-merge

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

Thanks!