chore: adds LDTransactionalFeatureStore, LDTransactionalDataSourceUpdates, and FDv2 DataSource impls.

[tanderson-ld]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

This PR moves changes for FDv2 from the temporary holding branch to main. All picked code has been previously reviewed and was picked from the temporary holding branch using file pick commands.

Resolved conflicts with @abarker-launchdarkly's recent initMetadata changes. Please verify those conflict resolutions @abarker-launchdarkly .

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[kinyoklion]

I have some concern that ordering is much more important now, so we may need to validate when callbacks can happen and when they can be applied.

In the old stuff you would have a stream and you know you will get puts and then later patches and the way the event source works we really aren't going to break that. Polling included the entire payload, so even http requests completing out of order wasn't a problem.

Now data could come from many sources that are less coordinated.