Remove newtonsoft, make the library native aot compatible

[idan-h]

Chore

Describe the chore

Bring supabase-csharp (and the sub-packages we use) up to .NET Native AOT so a dotnet publish -c Release -p:PublishAot=true build succeeds, and there are no runtime errors.

Acceptance criteria

• AOT publish finishes without linker errors or reflection warnings.
• App boots and runs basic Supabase flows (auth, simple CRUD) in the published binary.
• CI job produces an AOT artifact.

Steps

1. Add a repo-level Directory.Build.props with:

   <Project>
     <PropertyGroup>
       <PublishAot>true</PublishAot>
       <PublishTrimmed>true</PublishTrimmed>
       <InvariantGlobalization>true</InvariantGlobalization>
       <EnableDynamicLoading>false</EnableDynamicLoading>
     </PropertyGroup>
   </Project>

2. Swap Newtonsoft.Json for System.Text.Json + source-generated context.

3. Mark BaseModel with DynamicallyAccessedMembers and tag helper methods that use reflection with RequiresUnreferencedCode.

4. Replace Activator.CreateInstance calls with new T().

5. Run:

   dotnet restore
   dotnet build -c Release
   dotnet publish -c Release -p\:PublishAot=true -p\:SelfContained=true -o ./aot
   

   Fix any remaining trim warnings.

6. Update CI pipeline to run the publish step and cache the output.

Additional context

Reference the detailed checklist in docs/aot-supabase-notes.md.