Search backend: update searcher client to accept multiline matches
Created by: camdencheek
This updates the types in the searcher protocol to have an optional multiline return type. Right now, the client will accept both single-line or multiline results, re-interpreting single-line as the multiline type. Once we merge this and roll searcher out, we can update the server to actually send multiline matches.
Stacked on #35897
Test plan
This is currently unexercised, so I'm just depending on tests to catch any typos.