Minor: Allow extra argument to '/sites/%s/themes/mine' endpoint

[andres-blanco]

This PR is a follow up to this other one. The idea is to allow the argument dont_change_homepage when POSTing to /sites/%s/themes/mine.

While the name might seem strange is the one actually being sent by Calypso for Simple sites.

For more context see: paYKcK-1Mb-p2#comment-1395

Testing instructions:

1. Set up a Jurassic Ninja site.
2. Connect it to WP.com.
3. Connect to your JN site through ssh.
4. Add this snippet to the bottom of /srv/users/[USER]/apps/[USER]/public/wp-load.php.

add_action('jetpack_pre_switch_theme', function($arg1, $args ) {
        error_log( var_export( $arg1, true ) );
        error_log( var_export( $args, true ) );
}, 10, 2);

5. Run tail -F /srv/users/[USER]/apps/[USER]/public/wp-content/debug.log.
6. Go to the calypso.live link on this PR and select your JN site.
7. Go to the theme showcase.
8. Select the twenty twenty theme and press Activate
9. Verify this is the output in the log:

[12-Jul-2022 15:01:49 UTC] 'twentytwenty'
[12-Jul-2022 15:01:49 UTC] array (
  'theme' => 'twentytwenty',
  'dont_change_homepage' => [true|false], ### The value will depend on the radio button selected in the Activate modal
)

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

No.

Related to Automattic/wp-calypso#56869

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[markbiek]

LGTM!

[markbiek]

LGTM!