Add AffiliationPrefetcher to AffiliationService.

The new version of AffiliationPrefetcher won't be a KeyedService (as it
is not necessary) and will be owned by the AffiliationService.

This CL adds the AffiliationPrefetcher to the AffiliationService and
includes support in the service to register new affiliation sources,
which are sent to the prefetcher.

Bug: b/328037758
Change-Id: Ifcb4f47214660a9cbc6623268b9bfedfeb76f9f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5424659
Reviewed-by: Viktor Semeniuk <vsemeniuk@google.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Norge Vizcay <vizcay@google.com>
Cr-Commit-Position: refs/heads/main@{#1283941}