[BUG] [CSharp] httpclient does not support Options, only supports obsolete HttpRequestMessage.Properties

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Using openapi-generator to generate csharp clients with the HttpClient library ends up generating code that uses the obsolete HttpRequestMessage.Properties which was deprecated way back in .NET5

It should use the recommended Options instead.

Here's the warnings outputted by Visual Studio 2022:

1>C:\MyApi\Client\ApiClient.cs(418,17,418,35): warning CS0618: 'HttpRequestMessage.Properties' is obsolete: 'HttpRequestMessage.Properties has been deprecated. Use Options instead.'
1>C:\MyApi\Client\ApiClient.cs(505,39,505,53): warning CS0618: 'HttpRequestMessage.Properties' is obsolete: 'HttpRequestMessage.Properties has been deprecated. Use Options instead.'
1>C:\MyApi\Client\ApiClient.cs(505,87,505,101): warning CS0618: 'HttpRequestMessage.Properties' is obsolete: 'HttpRequestMessage.Properties has been deprecated. Use Options instead.'

Description

openapi-generator version

7.8.0

OpenAPI declaration file content or url

Here's the contents of the config file

generatorName: csharp
sourceFolder: Src
additionalProperties:
  library: httpclient
  targetFramework: net8.0

Generation Details

Steps to reproduce

• generate a client with csharp + httpclient + .NET8.0 using the openapi generator config pasted above,
• check the resulting code.

Related issues/PRs

• #12691

Suggest a fix

• Add support Options,
• add a csharp config option to toggle between HttpRequestMessage.Properties and Options, set HttpRequestMessage.Properties as default.

Start by generating the C# httpclient client with the provided config and inspect the generated ApiClient.cs locations cited in the warning output. Read docs/generators/csharp.md and related issue #12691 to understand the intended configuration behavior. Done means the generated .NET 8 client uses the supported request metadata API and no longer emits the reported obsolete-property warnings.