Store creation M2: site creation Networking & Yosemite layer changes

[jaclync]

Part of #8106

Description

After domain selection and before IAP, we need to create a free site and then pass the site ID for purchasing the eCommerce plan via IAP. This PR includes the Networking & Yosemite layer changes for creating a site, with parameters following the woocommerce.com/start flow on the web (the store configurations might change after the store becomes Atomic after IAP success). Sorry about the changes exceeding 500 diffs slightly 🙇🏻‍♀️ I thought it's easier to review both the remote and Yosemite store, especially for error handling and why we need to manually convert the site ID string from the Networking layer response to Int64 in Yosemite.

SiteAction was previously used in Yosemite unit tests, and I renamed it to MockSiteAction to avoid naming conflict.

Testing instructions

The new SiteAction.createSite isn't used in the app, just CI is sufficient. Integration with the store creation flow will be in a separate PR.

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

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[wpmobilebot]

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[itsmeichigo]

LGTM