Epic: Improve extension maintainability and experience

[codyrancher]

Create the APIs used to configure and initialize extensions before the app fully bootstraps itself

• Create the first API which demonstrates the structure and how the API will be initialized #8580
• Change extension initialization to occur after the rest of shell is initialized #8597
• Create the routing APIs #8582

Create reusable and composable layout components:

• Create a generic page header that can be used to create both standalone extensions and Rancher #8583
• Create the Rancher header using the generic header #8598
• Create a generic navigation component that can be used to implement the cluster explorer navigation #8599
• Create the cluster explorer navigation component using the generic navigation component #8600
• Create a generic ExtensionSlot component and configuration API #8649

[codyrancher]

Closing since we will only likely do 3 of these. One is done and two are still open and labeled with area/plugins