App Hosting Emulator: Init

[mathu97]

Properly initialize the App Hosting emulator when app hosting emulator is selected during firebase init emulators.

When the app hosting emulator is initialized ...

1. user should be prompted for root directory and rootDirectory config should be set
2. the default start command should be detected and startCommandOverride should be set
3. user should be prompted to export secrets

Additionally ...

• Refactored logic used by apphosting:config:export command so that it can be re-used at init.
• Added scaffolding to make it easy for emulators to add additional prompts when init emulators is ran.

[joehan]

Some nitty things, but lgtm after some small fixes.