Call HeartbeatController.LogHeartbeat from Auth's desktop implementation

[AlmostMatt]

Description

Update the LogHeartbeat method in auth_desktop to get an instance of HeartbeatController and call LogHeartbeat on that controller.
This PR also adds a static map from app_name to HeartbeatController instances and a static method to get such an instance. This is defined in app_desktop.h and implemented in app_desktop.cc
This PR creates app_desktop.h and moves an existing class definition from app_desktop.cc to app_desktop.h

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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.

Verified via unit test that by calling the static method in AppDesktop that a functional instance of HeartbeatController
Will manually perform end to end testing after code to add payload to auth requests is completed.

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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.
• [ x] 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 with FLAKINESS (succeeded after retry)

Requested by @AlmostMatt on commit c357d00
Last updated: Mon Aug 29 20:30 PDT 2022
View integration test log & download artifacts

Failures	Configs
auth	[TEST] [FLAKINESS] [iOS] [macos] [1/2 ios_device: ios_target] (1 failed tests)   FirebaseAuthTest.TestUpdateEmailAndPassword
firestore	[TEST] [FLAKINESS] [Android] [1/3 os: windows] [android_target] (1 failed tests)   ServerTimestampTest.TestServerTimestampsUsesPreviousValueFromLocalMutation
storage	[TEST] [FLAKINESS] [iOS] [macos] [1/2 ios_device: ios_target] (1 failed tests)   CRASH/TIMEOUT

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