Implement `LDClientRegisterFeatureFlagListenerUserData`

[ehsan]

This fixes #84. In this PR we're adding a version of the LaunchDarkly flag change listener APIs that allows the caller to pass a void * user data argument, which can be used similar to a closure in higher level languages. Also implement the C++ version of this API: registerFeatureFlagListenerUserData.

Requirements

• [x] I have added test coverage for new or changed functionality
• [x] I have followed the repository's pull request submission guidelines
• [ ] I have validated my changes against all supported platform versions I tested my changes on Linux. Don't have a Windows machine handy for testing there at the moment unfortunately.

Related issues

Provide links to any issues in this repository or elsewhere relating to this pull request. https://github.com/launchdarkly/c-client-sdk/issues/84

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[cwaldren-ld]

For anyone wishing for this feature, please see this update.