[Analytics Hub] Add support for Orders Analytics card data

[rachelmcr]

Support for #8149

Summary

The orders analytics card should be able to display two types of information: Total orders and Average Order Value. Both can be properly acquired directly from the Woo Core stats API.

Changes

We already have support to request this data in OrderStatsRemoteV4, mapping to the OrderStatsV4 model. This model already includes the Total orders data. However, we aren't parsing Average Order Value from this response.

We need to add the following support for getting and storing that data:

• Networking: Add avg_order_value to the OrderStatsV4Totals model.
• Storage: Add a new Core Data model with the average order value attribute added to the OrderStatsV4Totals storage entity.

[peril-woocommerce]

	Fails
🚫	Please add a feature label to this issue. e.g. 'feature: stats'

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