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

[bbartels]

@rajanadar Gentle ping on this :)

[ajacques]

+1 on AOT support. Importing a library that doesn't support AOT prevents the entire application from being able to enable AOT.

[stevefan1999-personal]

@bbartels Can I take your code and hard fork a new project? The name for VaultSharp will be changed to something else though

[bbartels]

Feel free :)

[bbartels]

@ezhevita @rajanadar @SergeyFilippov @hoerup Would be great to get a review :) It's not that AOT is necessary for VaultSharp itself, its more than any libraries that take a dependency on VaultSharp are blocked from proper AOT support. (This is our case for some internal libraries)

[SergeyFilippov]

@bbartels unfortunately i can't help you with getting approval, but majority of changes align perfectly with resolving limitation/requirement of AOT compilation and operation (as per PR description).

One single question I have is about .csproj file: you are overriding titles there, for the target frameworks that not yet supported (and if supported - should be added in a separate PR). Is this some missed leftovers from an internally supported version of this library?

[bbartels]

@bbartels unfortunately i can't help you with getting approval, but majority of changes align perfectly with resolving limitation/requirement of AOT compilation and operation (as per PR description).

One single question I have is about .csproj file: you are overriding titles there, for the target frameworks that not yet supported (and if supported - should be added in a separate PR). Is this some missed leftovers from an internally supported version of this library?

Fair point, removed those lines, thanks for the review!