[Analytics Hub] Add Networking support for Average Order Value analytics

[rachelmcr]

Part of: #8156

Description

This PR adds Networking support for retrieving the Average Order Value data from requests to GET /wc-analytics/reports/revenue/stats, to use on the Order Analytics card in Analytics Hub.

(We already make requests to this endpoint when dashboard stats load on the My Store screen but we aren't yet using Average Order Value in the app.)

Changes

• Adds the averageOrderValue property to the OrderStatsV4Totals model.
• Updates unit tests, mocks, etc to include this property.

Testing

1. Build and run the app.
2. When your dashboard stats load, confirm there are no decoding errors.

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.

[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 pr8157-5289894 on your iPhone

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[Ecarrion]

Looks good! Are you planning to update the storage layer in the next PR? If not I think the changes won't be reflected when fetched via the results controller.

[rachelmcr]

Are you planning to update the storage layer in the next PR?

Yep! #8162 updates the storage layer so we can store and fetch this data.