Add App Check support for Firestore

[a-maurice]

Description

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

Implements an App Check Credential Provider for Firestore to use for the desktop implementation, and hook into the iOS provider for the iOS implementation.
Adds a few tests to the App Check integration test to use Firestore.

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

Run the desktop tests locally.

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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.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[github-actions]

❌  Integration test FAILED

Requested by @a-maurice on commit d7ff369
Last updated: Mon Mar 13 18:44 PDT 2023
View integration test log & download artifacts

Failures	Configs
missing_log	[TEST] [ERROR] [iOS] [macos] [1/2 ios_device: ios_target]
admob	[BUILD] [ERROR] [iOS] [macos]
analytics	[BUILD] [ERROR] [iOS] [macos]
auth	[BUILD] [ERROR] [iOS] [macos]
database	[BUILD] [ERROR] [iOS] [macos]
dynamic_links	[BUILD] [ERROR] [iOS] [macos]
firestore	[BUILD] [ERROR] [iOS] [macos] [TEST] [FLAKINESS] [Android] [1/3 os: ubuntu] [1/2 android_device: android_target] (1 failed tests)   QueryTest.TestCanQueryWithAndWithoutDocumentKey
functions	[BUILD] [ERROR] [iOS] [macos]
gma	[BUILD] [ERROR] [iOS] [macos]
installations	[BUILD] [ERROR] [iOS] [macos]
messaging	[BUILD] [ERROR] [iOS] [macos]
remote_config	[BUILD] [ERROR] [iOS] [macos]
storage	[BUILD] [ERROR] [iOS] [macos]

Add flaky tests to go/fpl-cpp-flake-tracker

[dconeybe]

I've reviewed the firestore appcheck tests and the changes in the "firestore" directory. For the latter, I didn't look in too much detail, especially the code that looks to have been copied and adapted from the existing credentials provider. Is there anything I should look at in more detail?

[a-maurice]

I've reviewed the firestore appcheck tests and the changes in the "firestore" directory. For the latter, I didn't look in too much detail, especially the code that looks to have been copied and adapted from the existing credentials provider. Is there anything I should look at in more detail?

Yeah, it is mostly just copied from the AuthCredentialProvider logic, and just hooks into App Check instead. As long as that seems fine with you, that's good enough.

[dconeybe]

LGTM, for the parts that I reviewed. I'll withhold my approval unless you feel like my partial review is sufficient. But I think someone more familiar with the non-firestore parts of this PR should take a look too.

If possible, I'd like someone familiar with the non-firestore parts of the code to review those. I only looked at the firestore-specific parts.

[AlmostMatt]

Looks ok, just some minor comments