feat: Add experimental plugin support

[jsonbailey]

feat: Add experimental plugin support

Summary

This PR implements experimental plugin support for the Ruby Server SDK following the LaunchDarkly plugin specifications. The implementation adds a new Plugin interface that allows external integrations to register hooks and perform initialization tasks during SDK startup.

Key Changes:

• Added Interfaces::Plugins module with Plugin mixin and metadata classes (SdkMetadata, ApplicationMetadata, EnvironmentMetadata, PluginMetadata)
• Extended Config class to accept and validate plugins parameter
• Modified LDClient to collect plugin hooks during initialization and register plugins after data source setup
• Added comprehensive test suite covering plugin configuration, hook collection, registration, and error handling
• Added MockPlugin test utility following existing patterns

Implementation Details:

• Plugin hooks are collected before client initialization and appended after config hooks
• Plugins are registered in the order they were added to the configuration
• Error handling wraps plugin operations with logging to prevent SDK initialization failures
• Environment metadata provides plugins with SDK version, application info, and SDK key access

Review & Testing Checklist for Human

• Run the test suite - I was unable to run tests due to bundle install permission issues, so this needs verification that all tests pass
• Test plugin registration flow end-to-end - Create a simple plugin and verify it gets registered and its hooks are called correctly
• Verify no regression in existing functionality - Run existing hook tests to ensure plugin integration doesn't break current behavior
• Test error handling - Verify that plugin errors during get_hooks() and register() are caught and logged without breaking SDK initialization
• Validate against plugin specs - Double-check the implementation matches the plugin specification requirements exactly

Recommended Test Plan:

1. Create a minimal plugin that implements the Plugin mixin and registers a simple hook
2. Initialize the SDK with this plugin and verify the hook is executed
3. Test with a plugin that throws errors during registration to ensure graceful handling
4. Run existing test suite to ensure no regressions

---

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    Config["lib/ldclient-rb/config.rb"]:::major-edit --> LDClient["lib/ldclient-rb/ldclient.rb"]:::major-edit
    Interfaces["lib/ldclient-rb/interfaces.rb"]:::major-edit --> Config
    Interfaces --> LDClient
    LDClient --> DataSource["Data Source"]:::context
    LDClient --> Hooks["Hook System"]:::context
    
    TestSpec["spec/ldclient_plugins_spec.rb"]:::major-edit --> MockComponents["spec/mock_components.rb"]:::minor-edit
    TestSpec --> Interfaces
    
    Config --> |plugins parameter| PluginValidation["Plugin Validation"]:::context
    LDClient --> |get_plugin_hooks| PluginHooks["Plugin Hook Collection"]:::context
    LDClient --> |register_plugins| PluginRegistration["Plugin Registration"]:::context
    
    subgraph Legend
        L1["Major Edit"]:::major-edit
        L2["Minor Edit"]:::minor-edit  
        L3["Context/No Edit"]:::context
    end

    
    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF

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Notes

• Environment Issue: Bundle install failed due to permission errors, preventing test execution during development
• Implementation follows existing patterns: Used same error handling, logging, and validation patterns as existing hook system
• Plugin execution order: Config hooks execute first, then plugin hooks, then plugins are registered
• Session: Requested by @jsonbailey - Link to Devin run: https://app.devin.ai/sessions/6cac75e94159428e9e2e518957628365

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