[Analytics Hub] Fetch site stats for sessions card

[rachelmcr]

Closes: #8363, #8366

Description

This PR fetches site summary stats to use in the Sessions card in the Analytics Hub:

• Defines a period and quantity for the site stats request depending on the AnalyticsHubTimeRangeSelection.SelectionType.
• Adds a new async method to request the site stats from remote.
• Sets the sessions card loading state and the error state.
• Updates the view model binding so the card waits for both requests (the current period order stats and the site stats) before it binds the data.

Note: For now, the Sessions card will show a sync error when custom date ranges are selected. This behavior will be updated in #8367.

Testing instructions

1. Build and run the app.
2. Tap "See more" to open the Analytics hub.
3. Confirm the Sessions card loads with the expected data for the selected date range.
4. Disable your network connection.
5. Pull to refresh Analytics and confirm the card displays the expected error.

Screenshots

AnalyticsHub-SessionsCard.mp4

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

[peril-woocommerce]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 2 days Please, make sure to get it merged by then or assign it to a later expiring milestone

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[wpmobilebot]

You can test the changes from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr8412-b712073 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[ealeksandrov]

All good!