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 more of the structure in place.

Test Plan:
Viewed API console endpoint and read 20 pages of docs:

Made various calls: with query keys, constraints, pagination, and limits.

Viewed new Config → Modules page.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7715, T9964

Differential Revision: https://secure.phabricator.com/D14743