Onboarding to Jetpack upon saving the settings or upgrading the extension

[wjrosa]

See peNR48-1Qg#comment-1095-p2

Changes proposed in this Pull Request:

In this PR, we are introducing a mechanism to onboard plugin merchants to the Jetpack Transact Platform. It follows the same standard we have in PayPal, with most of the code ported from there.

The onboarding logic is triggered upon saving the Stripe settings or by performing a plugin upgrade.

Testing instructions

• Apply the 194549-ghe-Automattic/wpcom to your WPCOM Sandbox
• Checkout to this branch on your test environment (dev/jetpack-onboarding-class)
• Make sure it is externally accessible
• And that the /etc/hosts file contains an entry for your WPCOM sandbox:

<WPCOM Sandbox IP> public-api.wordpress.com

• Connect your Stripe account
• Attempt to save any Stripe setting, like toggling OCS on/off
• Check the value of the Stripe settings option on your database (wp_options, woocommerce_stripe_settings key)
• Confirm transact_onboarding_complete is present with the yes value

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
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