Make a DirectFromSellerSignals requester for worklets.

Cache requested results, and coalesce multiple requests for the same
resource to avoid making multiple Mojo calls to the network service for
the same resource.

To simplify the cache and reduce memory consumption, only cache one
response at a time.

Bug: 1186444,1320908
Change-Id: Id80f488139886899da5a97699972da65f538a8af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3913632
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1061636}