Investigate non-file based distrubuted eventing mechanism

Currently in Functions we leverage the fact that we have a shared file system as an eventing mechanism. For example, our "debug mode" relies on a file watcher on the debug_sentinel file watch mechanism.

In cases where we don't have a shared file system (e.g. Azure Functions on Mesh) we lack this capability. We need to investigate alternatives, and put these features behind interfaces so the implementation can be swapped out (we already do this to a degree).

Ideas include EventGrid, Azure Storage blob/polling, Redis Cache, etc.

Start by tracing the existing debug mode and its debug_sentinel file-watcher mechanism, including the interfaces already used for eventing. Compare the listed EventGrid, Azure Storage blob/polling, and Redis Cache alternatives, then define the interface boundary and document what is required for a swappable non-file implementation.

Written by the indexing model from the issue text.