Implement PrivateAggregationHost

Defines a mojo interface for worklets/renderers to request that an
aggregatable report be sent. Each request will be validated and then
passed on for budget approval. For now, this class is not used by or
integrated with the browser or any worklets/renderers.

To avoid type duplication/extra conversion, we migrate the aggregation
service to use mojo types for histogram contributions and the
aggregation mode.

Bug: 1323324
Change-Id: Idb04e1dba94ea6d75c29c3c3f99ac40ef3f51f41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3715540
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Alex Turner <alexmt@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Nan Lin <linnan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1018907}