feat: Add experimental plugin support

[jsonbailey]

feat: Add experimental plugin support

Summary

This PR implements experimental plugin support for the .NET Server SDK following the plugin specification. The implementation adds a complete plugin system including:

• Plugin base class: Abstract Plugin class with metadata, registration, and hooks support
• Metadata system: Classes for application, SDK, environment, and plugin metadata
• Configuration: PluginConfiguration and builder pattern for plugin setup
• LdClient integration: Plugin registration, hook collection, and environment metadata creation
• Comprehensive documentation: XML documentation for all plugin classes

The implementation follows established patterns from other LaunchDarkly SDK implementations and maintains backward compatibility.

Review & Testing Checklist for Human

• Plugin integration timing in LdClient: Verify that plugin registration happens at the correct time and doesn't interfere with existing initialization logic
• Configuration consistency: Confirm that the unification of config vs _configuration usage doesn't introduce regressions in configuration handling
• WrapperInfo property access: Validate that changes to WrapperInfo property names (Name/Version) are correct and don't break wrapper functionality
• End-to-end plugin functionality: Create a test plugin and verify it can be registered, provides metadata, and hooks are called correctly
• Error handling behavior: Ensure plugin error handling doesn't mask critical exceptions that should propagate to clients
• Memory and performance impact: Test with multiple plugins to ensure no memory leaks or significant performance degradation

Recommended test plan: Create a simple test plugin that implements all three methods (GetMetadata, Register, GetHooks), configure it via PluginConfigurationBuilder, and verify it integrates correctly with flag evaluations.

---

Diagram

graph TD
    LdClient[LdClient.cs]:::major-edit
    Plugin[Plugin.cs]:::major-edit
    PluginConfig[PluginConfiguration.cs]:::major-edit
    PluginBuilder[PluginConfigurationBuilder.cs]:::major-edit
    
    AppMeta[ApplicationMetadata.cs]:::major-edit
    SdkMeta[SdkMetadata.cs]:::major-edit
    EnvMeta[EnvironmentMetadata.cs]:::major-edit
    PluginMeta[PluginMetadata.cs]:::major-edit
    
    Components[Components.cs]:::minor-edit
    Config[Configuration.cs]:::minor-edit
    
    Hooks[Hook System]:::context
    EventProcessor[Event Processor]:::context
    DataSource[Data Source]:::context
    
    LdClient --> PluginConfig
    LdClient --> EnvMeta
    PluginConfig --> Plugin
    Plugin --> PluginMeta
    EnvMeta --> SdkMeta
    EnvMeta --> AppMeta
    PluginBuilder --> PluginConfig
    Components --> PluginConfig
    Config --> PluginConfig
    
    Plugin -.-> Hooks
    LdClient --> Hooks
    LdClient --> EventProcessor
    LdClient --> DataSource
    
    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

• All CI checks are passing including builds on Windows, Ubuntu, and macOS
• XML documentation has been completed for all plugin classes per GitHub review feedback
• Plugin registration includes comprehensive error handling to prevent individual plugin failures from breaking the client
• The implementation maintains backward compatibility - existing code will continue to work without any changes
• Environment metadata creation uses existing ApplicationInfo and WrapperInfo from the client context

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[jsonbailey]

The use of config and _configuration was mixed throughout this method. Unifying it to use the instance property once it is set.

[jsonbailey]

The CI builds will fail until we publish an updated common nuget package first. I wanted to get feedback before splitting the common work into its own PR.

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[kinyoklion]

I don't remember, do we use dotnet targets that support nullable reference types?

[jsonbailey]

Some do and some do not. Nullable reference types were introduced in C# 8 which is anything after 3.x and netstandard 2.1. We currently use C# 7.3 so that we can keep targeting netstandard 2.0 and net462.