Extract runtime APIs from AppHost

[felixb-wix]

AppHost contains many utility/administration functions. These functions can be extracted into APIs. They will be treated as any other API, and stop being special cases.

Below is proposed API names and contents.

AppHostDebugAPI

• getAllSlotKeys(): AnySlotKey[]
• getAllFeatures(): FeatureInfo[]

AppHostAdminAPI

• isFeatureInstalled(name: string): boolean
• isLazyFeature(name: string): boolean
• installFeatures(features: AnyFeature[]): void
• uninstallFeatures(names: string[]): void

[felixb-wix]

Here cfc516b I did first steps:

• create AppHostServices entry point
• create AppHostAPI (currently empty), which is contributed by AppHostServices entry point

Next step, the functions from AppHost need to be moved to AppHostAPI. Distinction between Admin and Debug API seems to be unnecessary.