R1:4ec6990ca767

Implement a rough initial version of ApplicationSearch-driven Conduit read endpoints

Summary:
Ref T9964. See that task for some context and discussion.

Ref T7715, which has the bigger picture here.

Basically, I want Conduit read endpoints to be full-power, ApplicationSearch-driven endpoints, so that applications can:

- Write one EditEngine and get web + conduit writes for free.
- Write one SearchEngine and get web + conduit reads for free.

I previously made some steps toward this, but this puts…