Add support for preview deployments internal alpha experiment 🧪

[JamesMGreene]

The GitHub Pages team has an 🤫 internal-only alpha experiment 🧪 running of a new feature for preview deployments (i.e. to enable a separate deployment of a Pages site per pull request).

In order to support our Hubbers in using that feature via this Pages deployment Action, we need to expose that functionality here. 📖

📣 Disclaimers: 👩🏻‍⚖️

• ⚠️ This feature is an internal alpha, and is thus only currently available internally to GitHub employees and/or GitHub-owned open source projects.
• 🛑 Sorry, but NO, you cannot currently use this feature if you do not meet the previously mentioned criteria. We have internal access controls in place to prevent unauthorized usage. 🔒
• 📆 We do NOT currently have any estimated ship date for this feature. It is still possible that it may never ship to general users 😱, but we do hope to make it generally available someday in the future. 🤞🏻

[JirawatChanthamail]

J

[joshnatis]

Hi James, leaving a note here that our college's open source club is super excited about this feature :). We hope to use it in our syllabi directory project, https://github.com/Hunter-Open-Source-Club/syllabi, to help contributing students preview their contributions without having to run anything locally (and similarly, to help us review contributions without needing to sync them to our computers!).

Thanks for your hard work on this.