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

[rachelmcr]

Closes: #8156
⚠️ Depends on #8157 ⚠️

Description

This PR adds Storage 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 and store the data when dashboard stats load on the My Store screen but we aren't yet using Average Order Value in the app.)

Changes

• Adds Core Data Model 78.
• Adds averageOrderValue attribute to OrderStatsV4Totals storage entity.
• Adds migration test for Model 77 to 78 migration.

Testing

1. Build and run the app and confirm dashboard stats load.
2. Replace a build from trunk with this build and confirm the migration works with no Core Data errors or warnings.

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 pr8162-008f233 on your iPhone

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

[ealeksandrov]

LGTM

nit: StatsStoreV4Tests needs an update for averageOrderValue in samples.

[peril-woocommerce]

	Warnings
⚠️	PR has more than 500 lines of code changing. Consider splitting into smaller PRs if possible.

Generated by 🚫 dangerJS

[rachelmcr]

nit: StatsStoreV4Tests needs an update for averageOrderValue in samples.

Oops, yep, fixed that in 008f233. Thank you!