[CodeHealth] Use `optional<string>` in quick_answers

This CL migrates several places in quick_answers where
`std::unique_ptr<std::string>` were being used in conjunction with
`UrlLoader` callbacks, to the interface relying on a `std::optional`.
The `unique_ptr` in interface has been marked for deprecation for a
while now.

Bug: 410044687
Change-Id: Ie39520a0d98c7c3b5a7cedb8f1e4afb649266356
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6644531
Commit-Queue: Claudio DeSouza <cdesouza@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1478451}