Dynamically invoke correct trigger when running functions emulator with --inspect-functions flag

[taeold]

In #4149, we made large refactor of the Functions Runtime. Namely:

1. We no longer relied on Functions Runtime to parse triggers. Instead, we use RuntimeDelegate, the same procedure used to parse trigger in firebase deploy command.

2. Each process running the Functions Runtime was bound to a specific trigger.

Unfortunately, this change broke support for --inspect-function where users can attach Node Debugger to step through their function when triggered.

The debugging experience supported w/ --inspect-function relies on the fact that a single process executes all function triggers. This is drastically different to how functions run in Production environment, but it is a very useful feature that makes it easy to step through all functions in a single debug session.

I wasn't aware of the debugging capabilities when making the refactor. Unfortunately, the debug feature cuts across a strong assumption I've made for the future of Functions Emulator. This is a rather hasty rollback - I'm going to have to think more deeply about how we want the evolve this feature.

Fixes #4189, #4166