chore: Create FDv2 File Data Source

[jsonbailey]

Note

Medium Risk
Introduces a new FDv2 initializer/synchronizer that reads and auto-reloads flag/segment data from local files, adding threading and file-watching behavior that could affect runtime stability if used incorrectly.

Overview
Adds a new FDv2-compatible file data source (FileDataSourceV2) that can act as both a data system Initializer (fetch) and Synchronizer (sync), loading flags/segments from JSON/YAML files and emitting ChangeSet updates when files change.

Exposes this via a new public LaunchDarkly::Integrations::FileData.data_source_v2 builder (with paths and poll_interval), implements optional file watching via the listen gem with a polling fallback, and includes a comprehensive new spec suite covering load/validation, change detection, and shutdown behavior. Also updates .gitignore to ignore /vendor/bundle.

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