[WebBundle] [WIP] Extendable backend menu

 Q | A
| --- | ---
| Bug fix? | no
| New feature? | yes
| BC breaks? | no
| Deprecations? | no
| Tests pass? | n/a
| Fixed tickets | n/a
| License | MIT
| Doc PR | n/a

This feature enables to extend the backend menu subsection (assortment, sales, ...) with custom menu entries via tagged services. The advantage of the compiler pass/tags are, any bundle can create his menu items builder and tag them, but a user of this bundle is not required to depend on the WebBundle too. So the backend menu integration becomes optional.

- [ ] Needs review and feedback
- [ ] Add tests for this feature (need help which)
- [ ] Submit PR for the documentation