[Remote Config] Add Custom Signals

[a-maurice]

Description

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

Add support for setting Custom Signals in Remote Config. Note that this depends on firebase/firebase-cpp-sdk#1899

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Testing

Describe how you've tested these changes.

Running the tests locally on Android/iOS/Mac.

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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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Feature Request:
#1210

[gemini-code-assist]

Code Review

This pull request adds support for setting Custom Signals in Remote Config, including C# API bindings, SWIG interface updates, and corresponding UI and automated test cases. The review feedback highlights two important improvements: first, explicitly disposing of SWIG-wrapped StringList and VariantList objects in SetCustomSignalsAsync to prevent unmanaged memory leaks, along with adding a defensive check against null keys; second, resolving a potential race condition in the automated test by directly awaiting both FetchAsync and ActivateAsync instead of relying on FetchDataAsync which does not await activation.