Part of: #8363

Description

This PR adds support in the Networking layer for getting views (in addition to visitors) when requesting SiteVisitStats from the API (the "visits" endpoint: GET /rest/v1.1/sites/{SITE}/stats/visits).

This allows us to get the views and visitors stats we need in the Analytics Hub for the quarter time period. We can get these stats for other time periods from the "summary" endpoint added in #8372, but that endpoint doesn't support quarters.

Instead, for quarters we can make a request to this "visits" endpoint to get the stats for every month in the quarter, and sum the monthly stats to get the quarterly totals.

We currently store SiteVisitStats in Core Data; support for views in the Storage layer will be added in a following PR.

Changes

1. Updates the request parameters in SiteStatsRemote to request both the visitors and views fields from this endpoint.
2. Adds views to the SiteVisitStatsItem model (and updates SiteVisitStats to decode them).
3. Updates the related fakes, copiables, mocks, and unit tests to include views.

Testing

Views aren't yet stored or used in the app, so make sure unit tests have expected changes and still pass.

Submitter Checklist

Update release notes:

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.