Add Native AOT support for .NET 8+

[bbartels]

Summary

• Add System.Text.Json source generation via VaultSharpJsonContext to enable Native AOT compilation on .NET 8+, replacing all anonymous types used as request payloads with strongly-typed models (AotRequestModels.cs) decorated with [JsonPropertyName] attributes
• Switch serialization/deserialization calls to use JsonTypeInfo-based overloads on .NET 8+ (via #if NET8_0_OR_GREATER) while preserving reflection-based fallback for older target frameworks
• Update all [JsonConverter(typeof(JsonStringEnumConverter))] enum attributes to use the AOT-compatible generic JsonStringEnumConverter on .NET 8+
• Refactor VaultApiException to use JsonDocument-based parsing instead of JsonSerializer.Deserialize<Dictionary<string, IEnumerable>>, avoiding reflection
• Remove unnecessary [JsonDerivedType] attributes from AbstractAuditBackend (already handled by custom AuditBackendJsonConverter) and add missing [JsonPropertyName("type")] to audit backend subtypes
• Expose JsonSerializerOptions on VaultClientSettings to allow consumers to register custom type resolvers (e.g., for KV secret data types)

[bbartels]

@rajanadar If you get a chance to take a look! Would be great to expand with support across the dotnet ecosystem. We have some apps using Vaultsharp which we'd love to publish as AOT instead