Send Email Preview Endpoint

[lezama]

Partially Fixes #31021

Proposed changes:

• Introduces and endpoint that sends and email preview given a post id to the email of the current user.
• Adds a Trait that could be reused on any endpoint that needs to proxy requests to wpcom.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

code-D110864
p2-peKye1-6z

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Run the new tests jetpack docker phpunit -- --filter=WP_Test_WPCOM_REST_API_V2_Endpoint_Send_Email_Preview
• Apply bin/jetpack-downloader test jetpack add/preview-email-endpoint and D110864 to your wpcom sandbox.
• Sandbox public-api
• Use https://developer.wordpress.com/docs/api/console/ to test the new endpoint.
• Use the client side Add Send Email Preview feature #31021 to test in your self-hosted site following the instructions in code-D110864
• Thanks!

[github-actions]

Are you an Automattician? You can now test your Pull Request on WordPress.com. On your sandbox, run

bin/jetpack-downloader test jetpack add/preview-email-endpoint

to get started. More details: p9dueE-5Nn-p2

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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• ⚠️ All commits were linted before commit.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
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• ✅ Add changelog entries to affected projects
  

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Jetpack plugin:

• Next scheduled release: July 5, 2023.
• Scheduled code freeze: June 26, 2023.