[EuiHeader] Added sections and position props

[cchaos]

Continuing support for new nav updates

Sections

One of the most complex components to build is the EuiHeader. There are 6 different components you have to weave together the right way to display in the header.

This PR is an attempt to simplify a portion of that by allowing the user to pass sections instead of children.

The docs example basically spits out the same content as the example above it but in 20 less lines of code (where it matters: in the render).

The passed prop looks something like this:

sections={[
  {
    items: [...],
    borders: 'right',
    breadcrumbs: [...],
  },
  {
    items: [...],
  },
]}

Where items is just an array of nodes, and breadcrumbs is an array of breadcrumbs. All the truncation should work properly and each section is spaced via space-between.

---

Position

I also decided that we should be managing the fixed position since we also know the height of the header. So I added the prop position which defaults to static but if changed to fixed will also apply a class to the window body with the correct padding.

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in IE11 and Firefox
• Props have proper autodocs
• Added documentation examples
• Added or updated jest tests
• [ ] Checked for breaking changes and labeled appropriately
• [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2928/

[cchaos]

Hmm, don't review this PR just yet, I've got a truncation issue with the breadcrumbs.

[cchaos]

I take this one back for now, I want to rethink it some more. This is originally just the quick way I did it in the K8 POC, but I don't think it's the right solution. Since I can't put into draft status, it is in WIP status.

[cchaos]

Ok, I think I came up with a better solution and I've pulled this back into read for review status.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2928/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2928/

[cchaos]

I really need to start making sure I push my changes before I create or update PR's. I'm really sorrry @thompsongl & @snide if you've already starting looking at this, but apparently I never pushed my last commit which actually fixes it from my last attempt. It is now up to date. 😑

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2928/

[cchaos]

jenkins, test this

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2928/

[thompsongl]

Code changes LGTM

Need to rebase or merge master and update the CL entry

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2928/

[miukimiu]

I reviewed the code and the build preview and everything LGTM! 🎉

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_2928/