Multisite: Add filter to allow custom form attributes in new site creation

[sakibmoon]

This change introduces the network_site_new_form_attributes filter to the site creation form (/wp-admin/network/site-new.php) in WordPress Multisite.

The filter allows developers to modify form attributes such as enctype="multipart/form-data", enabling file uploads or other custom behaviors when creating a new site.

This enhancement provides more flexibility for developers extending the multisite functionality without requiring modifications to core files.

Trac ticket: https://core.trac.wordpress.org/ticket/62086

---

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props sakibmoon, realloc.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• The Plugin and Theme Directories cannot be accessed within Playground.
• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[lloc]

@sakibmoon There might be some room to improve this PR.

1. If the filter returns something like autocomplete="off", it will be appended directly after the class attribute, resulting in:

<form method="post" action="site-new.php" class="wp-validate"autocomplete="off">

Which is invalid HTML due to the missing space. You could add a check to prepend a space if needed.

2. It might also be worth considering some sanitization or validation of the filtered output — perhaps passing it through something like wp_kses_attr_check() or at least documenting that the output should be properly escaped HTML attributes.

Thanks for your work on this!

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 60398
GitHub commit: fb4ec6f

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.