Desmith/alt api endpoints

[dsmith4-godaddy]

Initial support for overriding api.wordpress.org as the location for "WordPress API" requests, most commonly used for theme/plugin updates.

This doesn't touch downloads themselves - the assumption is that your new API will modify responses, to direct downloads to a suitable new location.

Trac ticket: https://core.trac.wordpress.org/ticket/62132

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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]

Hi @dsmith4-godaddy! 👋

Thank you for your contribution to WordPress! 💖

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[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.

[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.

Unlinked Accounts

The following contributors have not linked their GitHub and WordPress.org accounts: @dsmith4-godaddy.

Contributors, please read how to link your accounts to ensure your work is properly credited in WordPress releases.

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

[dsmith4-godaddy]

(Cannot link this Github account, because wordpress.org evidently won't allow me to link multiple GH accounts to a profile. Sigh.)