[PriceInsights][iOS] Add histograms to Price Insights Model.

This CL adds four histograms to the Price Insights Model.
- IOS.PriceInsights.Model.ProductInfo to record ProductInfo availability
- IOS.PriceInsights.Model.PriceInsightsInfo to record PriceInsightsInfo availability
- IOS.PriceInsights.Model.CanTrack to record if the page can be tracked
- IOS.PriceInsights.Model.IsSubscribed to record whether or not the page is being tracked.

Bug: chromium:326450366
Change-Id: I40649532feccb79695987ddec450d8b1bce2832e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5783843


Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Gayane Petrosyan <gayane@google.com>
Commit-Queue: Cheick Cisse <cheickcisse@google.com>
Auto-Submit: Cheick Cisse <cheickcisse@google.com>
Cr-Commit-Position: refs/heads/main@{#1343911}