Site Migration: Persist from param through flow, pass to Atomic transfer API

[sixhours]

Related to #98937 and a redo of #98938 which was reverted due to test confusion.

Proposed Changes

• Rather than attempting to save the migration_source_site_domain site option from the front end, pass it through the DIY flow and to the Atomic site transfer API so we can set it on the back end.
• This should also fix Hosted Site Migration: DYF Instructions is not showing direct links to the plugin installation #98682 since we're passing the from query param through checkout to the migration instructions step.

Why are these changes being made?

• paYKcK-5KE-p2

Testing Instructions

• Sandbox public-api.wordpress.com
• Switch to this PR/use calypso.live
• Go through the migration flow from start: choose Import or Migrate at the goals step, put in your external WordPress test site URL, choose Migrate, choose Do it myself, upgrade/checkout.
• Grab the site ID for the test site you've created.
• Via wpsh on your sandbox, check the value of migration_source_site_domain for your new test site ID:

> switch_to_blog( $test_site_id );
> echo get_option( 'migration_source_site_domain' );

• The returned value should be the URL you input when you went through the migration flow.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-131565

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-131565&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-131565&env=a8c-for-agencies

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug add/from-param-to-flow-again on your sandbox.