Update for new header

[janfaracik]

This is very (and I mean very) rough, want to get some thoughts on this early version for the new Jenkins header.

The thinking behind this PR is to minimise the need for completely overriding the header, instead relying on replaceable slots in core's header. This will make it easier to update core in the future, without risk of breaking/impacting this plugin.

I've also added some examples themes to the plugin, demonstrating it's customizability and also making it easy for users to revert to the classic header theme if they prefer.

Still need to add the context aware logo.

Testing done

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue