Hide some private friend functions from swig

[a-maurice]

Description

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

Hide some private friend methods from swig, since it is trying to generate C# based on them, when it shouldn't be. This is because a new version of SWIG came out that does this.

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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.

https://github.com/firebase/firebase-unity-sdk/actions/runs/8563655507

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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 with FLAKINESS (succeeded after retry)

Requested by @a-maurice on commit 6bbda6a
Last updated: Fri Apr 5 17:11 PDT 2024
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [FLAKINESS] [Android] [1/3 os: ubuntu] [1/2 android_device: emulator_ftl_target] (1 failed tests)   QueryTest.TestCanExplicitlySortByDocumentId [TEST] [FLAKINESS] [iOS] [macos] [1/2 ios_device: ios_target] (6 failed tests)   ServerTimestampTest.TestServerTimestampsCanReturnEstimatedValue   ServerTimestampTest.TestServerTimestampsCanReturnPreviousValue   ServerTimestampTest.TestServerTimestampsWorkViaSet   ServerTimestampTest.TestServerTimestampsWorkViaTransactionSet   ServerTimestampTest.TestServerTimestampsWorkViaTransactionUpdate   ServerTimestampTest.TestServerTimestampsWorkViaUpdate
storage	[TEST] [FLAKINESS] [Android] [1/3 os: macos] [1/2 android_device: emulator_ftl_target] (1 failed tests)   CRASH/TIMEOUT

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