Support defining functions in code instead of function.json

[ejizba]

This work was unblocked in the host by Azure/azure-functions-host#7606. For now, it requires users to include EnableWorkerIndexing in the AzureWebJobsFeatureFlags app setting.

This specific issue is to track basic support in the core worker api for defining functions in code. I imagine this essentially just means users can copy/paste their current "function.json" content into a js/ts file. It should be a simple change to their codebase that will let them completely redefine their file structure. Anything fancier will be tracked by the new programming model package in #480

Prereqs:

• Provide api for the worker: Publish types package for core api #567
• Add entrypoint for app-level code: Add entrypoint for app-level code #537

[ejizba]

I did a rough first commit on this branch that only works with "OptionA-Simple" in the prototype repo here. Keep in mind you have to add this to the local.settings.json in the app you're testing:
"AzureWebJobsFeatureFlags": "EnableWorkerIndexing"

Plan as discussed today was to have @hossam-nasr take it from here, which includes:

1. Get the other prototype options to "work" - for the purposes of a user study and to validate the core api is on the right track
2. Make the rough commit an actual PR for merging