Add the public App Check API files

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Adds the files that make up most of the public App Check API (minus the built-in providers). Most of the functionality is not implemented, that will be in follow up PRs.

---

Testing

Describe how you've tested these changes.

It builds locally.

---

Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

---