feat: Add plugin support to Client SDK

[abelonogov-ld]

Summary

This PR adds plugin architecture and hook-based flag evaluation lifecycle support to the LaunchDarkly Client SDK for .NET, ported from the Server SDK implementation.

Plugin System

• Introduced a Plugin base class (Plugins/Plugin.cs) that provides a GetHooks() method, allowing plugins to register hooks with the SDK
• Added PluginConfiguration and PluginConfigurationBuilder to configure plugins via ConfigurationBuilder.Plugins()
• Updated LdClient to initialize plugins during startup, collect their hooks, and dispose of them properly during shutdown
• Refactored LdClient cleanup logic to ensure proper disposal order of resources

Hook Framework

• Hook base class with virtual BeforeEvaluation, AfterEvaluation, BeforeIdentify, and AfterIdentify stage methods, plus IDisposable support
• EvaluationSeriesContext - captures flag key, context, default value, method, and environment ID for evaluation hooks
• IdentifySeriesContext / IdentifySeriesResult - context and result types for identify hooks
• Method constants mapping to SDK variation methods (e.g. LdClient.BoolVariation)
• SeriesDataBuilder - fluent API for building immutable series data passed between hook stages

Hook Execution Engine

• Executor / NoopExecutor implementing IHookExecutor - manages hook lifecycle with proper forward/reverse ordering
• BeforeEvaluation / AfterEvaluation stage executors with error isolation - exceptions in one hook do not prevent other hooks from executing
• Integrated hook execution into LdClient.EvaluateInternal(), wrapping flag evaluations with BeforeEvaluation and AfterEvaluation calls
• Added GetMethodName<T>() to resolve the correct Method constant based on variation type and detail mode
• Fixed prerequisite evaluation to use EvaluateInternal directly, avoiding recursive hook invocations

Test Plan

• EvaluationSeriesTest - hooks execute in LIFO order (forward before, reverse after)
• EvaluationSeriesTest - exceptions in hook stages don't prevent other hooks from running
• EvaluationSeriesTest - stage failures log expected error messages with flag key and hook name
• EvaluationSeriesTest - disposing executor disposes all registered hooks
• PluginConfigurationBuilderTest - plugin configuration builder tests pass
• LdClientPluginTests - end-to-end plugin/hook integration tests pass
• All 7 evaluation series tests pass (dotnet test --filter EvaluationSeriesTest)

---

Note

Medium Risk
Touches core LdClient evaluation flow by wrapping all Variation* calls with hook execution and adds plugin registration at startup, so regressions could affect flag evaluation behavior/performance despite safeguards like noop execution when unconfigured.

Overview
Adds client-side plugin support by introducing PluginConfigurationBuilder/PluginConfiguration and wiring a new Plugins(...) option through Components, ConfigurationBuilder, and Configuration.

Integrates plugin lifecycle into LdClient initialization: builds plugin config, creates EnvironmentMetadata, collects plugin-provided Hooks, instantiates a hook Executor (or NoopExecutor when none), then registers plugins; hook execution is disposed with the client and logs under a new LogNames.HooksSubLog.

Introduces a hook API and execution pipeline for evaluations (Hook, EvaluationSeriesContext, stage executors) and wraps VariationInternal so hooks run before/after evaluations; prerequisite evaluations are refactored to call EvaluateInternal directly to avoid triggering hooks. Adds unit tests covering hook ordering/error isolation/disposal and plugin builder/registration behavior.

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Example of API

var ldConfig = Configuration.Builder(mobileKey, LaunchDarkly.Sdk.Client.ConfigurationBuilder.AutoEnvAttributes.Enabled)
        .Plugins(new PluginConfigurationBuilder()
        	.Add(ObservabilityPlugin.Builder(new ObservabilityOptions(
        		isEnabled: true,
        		serviceName: "maui-sample-app",
				otlpEndpoint: "https://otel.observability.ld-stg.launchdarkly.com:4318/",
				backendUrl: "https://pub.observability.ld-stg.launchdarkly.com/"
        	)).Build())
        	.Add(SessionReplayPlugin.Builder(new SessionReplayOptions(
        		isEnabled: true,
        		privacy: new SessionReplayOptions.PrivacyOptions(
        			maskTextInputs: true,
        			maskWebViews: false,
        			maskLabels: false
        		)
        	)).Build())
        ).Build();
        
        

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