Products Onboarding: Add support for using frame_nonce in product previews

[rachelmcr]

Part of: #8046

Description

This PR includes the support @ealeksandrov added for a site's frame_nonce option in the Networking, Yosemite, and Storage layers:

• Gets the frame_nonce site option when the user's sites are requested from the me/sites endpoint.
• Stores the value as the frameNonce attribute on the Site entity in Core Data (adding a new Core Data model).

Testing

The frameNonce attribute is not yet used in the app; confirm all unit tests 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.

[peril-woocommerce]

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

Generated by 🚫 dangerJS

[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 pr8052-78be945 on your iPhone

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[pachlava]

I was automatically added as a reviewer as a part of "mobile-ui-testing-squad" because the PR changes a UI tests related file: namely, adds (78be945) frame_nonce param to rest_v11_me_sites.json.

I can see that all UI tests started to pass after that, so it's good to go from our side. Kudos for doing this @rachelmcr! 🙌

(I did not do a code review of the rest of the files).

[rachelmcr]

Just a note that I'm waiting to merge this PR until we've decided whether to move forward with storing frame_nonce in Core Data or another approach. (Internal ref: pe5pgL-1ea-p2)

[ealeksandrov]

LGTM!
As we discussed in post linked above, I consider it safe to merge
Thanks for taking another look around frame-nonce logic in Jetpack!