feat: Make common sdk AOT safe

[justindbaur]

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

Related issues

Part of #90
Prerequisite for launchdarkly/dotnet-sdk-internal#49

Describe the solution you've provided

I made all targets of the common SDK trim safe using the built in system text json source generators.

Describe alternatives you've considered

The already written converters could be called directly in the places where LdValue and Context need to be serialized.

Additional context

I do have a test console app that can be published with AOT and then ran to make sure that these items can be serialized properly but wasn't totally sure where I should put it. If that is desired I can add it, just need to know where to put it.

---

Note

Adopts System.Text.Json source generation for Context/LdValue and updates serializers, converters, and project settings to be AOT/trim friendly.

• Serialization (NET7+):
  • Use generated context LdJsonSerializerContext for Context.ToString, LdValue.Parse, and LdValue.ToJsonString.
  • Add LdJsonSerializerContext with [JsonSerializable] for LdValue and Context.
  • Annotate LdJsonSerialization methods with RequiresUnreferencedCode/RequiresDynamicCode warnings and messages.
• JSON Converters:
  • Use Enum.GetValues<T>() on NET5+ for BigSegmentsStatus, EvaluationErrorKind, and EvaluationReasonKind identifier mapping.
  • Correct enum iteration in EvaluationReasonKindConverter to use the proper enum type.
• Project Configuration:
  • Conditionally set LangVersion to 9.0 and IsAotCompatible true for NET7+ targets.
• Behavior:
  • Maintain existing paths for older frameworks via #if guards.

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[jsonbailey]

@justindbaur thanks for submitting the PR. I will review this as soon as I can.